Tip: See my list of the 
MostEsperanto is the most widely spoken constructed language (unless you count Modern Hebrew as a constructed language). In this article, I will try to shed some light on the pros and cons of learning Esperanto from an objective point of view.
The good
Esperanto is an extremely easy language to learn
Esperanto has a relatively simple grammar, which is also completely regular (there are no exceptions). There are no irregular past tenses, no irregular plurals, no irregularly used prepositions… Additionally, the pronunciation is easy, and the writing system is completely phonetic.
Esperanto has a completely regular way of deriving new words from the ones you already have. To negate a word in English, we use various prefixes like non-, un-, in-, im-, ir-, il-, dis-, and more, and a non-native speaker has to remember when to use which (saying “nonpossible” or “disnecessary” will not do the trick). In Esperanto, you just use ne- for all of them. Hundreds of English prefixes and suffixes can be expressed with just a few dozen unambiguous Esperanto ones.
To understand why this is such a big deal, let’s take a look at an example. Granda means “large” in Esperanto (think of “grand” in English). The suffix -igi means “to make, to cause to be”, so grandigi means “make (something) large”. The prefix pli- means “more” (think of “plus”), so pligrandigi means “to enlarge”. Does the English prefix “en-” always work like that? Not at all; we cannot say “enfast” or “enbetter”, but in Esperanto, we can take the word rapida “fast” and form plirapidigi “to accelerate” or bona “good” and form plibonigi “to improve”.
The point is that you can combine prefixes and suffixes with words you already know whenever the results make sense. Instead of learning thousands of dissimilar words like “fast – speed – accelerate”, you essentially need only a few hundred roots and use a few dozen affixes like “rapid·a – rapid·eco – pli·rapid·igi” to become proficient in the language.
Last but not least, the root words used in Esperanto were mostly chosen in such a way that they are easy to recognize and easy to remember for speakers of any European language (as you might have noticed from the examples above). All in all, practice has shown the following fact:
Reaching proficiency in German may take between 3 to 10 years, depending on how active you are, whereas becoming fluent in Esperanto will take about three months up to a year, with the same amount of effort.
Esperanto is a great language if you want to learn a second language just for the sake of experiencing the process of language learning
Some people want to learn Esperanto just for the sake of learning some foreign language. It is quite easy to find motivation to learn Spanish for a native English speaker living in the US, simply because there are so many Spanish speakers around. But for an Australian, for example, learning any foreign language isn’t really that useful.
If you decide to learn a foreign language just in order to experience the process of language learning, learning Esperanto may be a great start. It will be the easiest second language for you to learn, at least among languages that are currently in use. And if you eventually decide to learn another natural language, learning Esperanto first will make the transition less daunting.
Esperanto is a great “stepping stone” if you want to learn other European languages
Had Esperanto been constructed today, I believe its vocabulary would have looked slightly different. It contains quite a few roots (but still only a minority) that will not be immediately comprehensible to an English speaker, e.g. tago, from German Tag, meaning “day” (a word like “dejo”, pronounced “day-oh”, would have been a better choice because speakers of English as well as many Romance languages would find it intelligible).
While this makes Esperanto a tiny bit harder to learn than it theoretically could have been, it also introduces a small amount of vocabulary from various European languages in a context that is easy to understand (and hence easy to remember). If you learn Esperanto first and then move on to another European language, it will make the transition less overwhelming than if you directly started learning another natural language—and if you already speak more than one European language, you will likely understand the vast majority of Esperanto roots immediately.
Esperanto may actually help you learn other languages faster
Strange as it may seem, if you are monolingual and invest a year in learning Esperanto and then four years in learning French, you may end up speaking better French than if you just learned French for five years straight (really, I am not making this up; there have been studies on children proving that).
How is that possible? Esperanto will teach you grammatical concepts (such as how to use various tenses, prefixes, endings, etc.) in a pure and easy-to-remember way. Grammatical concepts are always obscured by irregularities in natural languages, and it may take a lot of time to understand the same underlying principles without being given any clear examples.
It is like learning to play the piano—it will take less time overall if you master the basic principles with easy études before playing harder compositions. What also plays a role is that learning Esperanto will teach you how to learn foreign languages. You will better understand how your own brain works, which will, again, make learning another language more efficient.
To be honest, I believe that this effect is caused mainly by the fact that foreign languages are not being taught well. Teachers explain grammatical concepts without providing any intuitive insight, and students memorize conjugation tables and lists of exceptions without being able to use them in fluent speech. If the teaching methods used to teach foreign languages were better, this so-called propaedeutic value of Esperanto would mostly disappear, in my opinion. But until then, Esperanto may be useful for beginners.
Learning Esperanto may help you find new friends, if that is your thing
There is an enthusiastic (though small) community of Esperanto speakers in virtually every bigger city in the world. In this respect, Esperanto is a hobby, just like playing chess or skiing. Esperanto speakers often emphasize that Esperanto allowed them to make lots of new friends; they like the fact that, unlike other hobbies, Esperanto has the inherent advantage of being a means of communication as well as a hobby (you would hardly be able to discuss strategic ideas in chess if you are not able to communicate in a common language).
Whether finding a hobby whose main benefit is the ability to make friends almost everywhere in the world is the right thing for you depends on your nature (as an introvert, I would not count that as one of the perks). If you are looking for a more practical hobby, learning some other natural language (like German or French) would be objectively more useful than learning Esperanto, but you must also be ready to spend significantly more time before they start being useful.
It feels good to understand Esperanto
One thing is for sure: Being able to understand Esperanto feels good. The reason is that although it looks somewhat exotic at first sight, it is really easy to learn. For example, the following sentence is taken from Wikipedia:
It looks like something hardly comprehensible, and yet one can manage to learn the language many times faster than languages which look much easier at first sight (because what makes Esperanto look strange are mostly the prefixes and suffixes, but those also make it really easy to understand when you learn what they mean). When you finally read a sentence like this one and understand it without trouble (which is a level of proficiency you can reach in just a few weeks of intense learning), it just feels good.
The bad
No objective discussion would be complete without discussing also the negatives. Many passionate Esperanto speakers would try to convince you that there are none, but not everything about Esperanto is that great:
Language is a means of communication, and Esperanto doesn’t have many speakers
For me, the primary purpose of any language is communication. Every language I have learned allows me to communicate with more people than I could before, either actively or passively. Even just seeing Facebook updates of my friends in their native languages and being able to understand them is a satisfying experience.
The fact is that only a tiny fraction of the world’s population (only about 2 million people) are able to speak Esperanto (and most of them are not even fluent in it). It doesn’t reach (by far) even the top 100 spoken languages.
Speakers of Esperanto are scattered throughout the world, and the vast majority of them already speak another common natural language, such as English, German, Spanish, or Russian. The probability of Esperanto being the only means of communication with just a single person in my life is tiny, unless I specifically try to find speakers of Esperanto.
In other words, I’ve heard the question Sprechen Sie Deutsch? or Parlez-vous français? many times from random strangers, but I have yet to meet a person asking me Ĉu vi parolas Esperanton?
If you want to learn a language to expand the number of people with whom you are able to communicate, a language like Spanish would be a much better choice (there are over 400 million native Spanish speakers, most of whom are not proficient in English… or Esperanto).
Language is bound to a distinct culture and history, and Esperanto doesn’t have much of it
Learning a new language grants you access to a whole new culture. There is a lot of literature, a great part of which has probably never been translated into a language you already speak (also, reading a translation is never the same as reading the original). There will be many new films to watch. There will be a wholly new cuisine to appreciate and to speak about in the language.
One cannot really say that about Esperanto. There are just a few hundred original novels written in Esperanto (not counting translations) and virtually no films shot in it. I somehow feel that reading Russian classics while being able to discuss them with 150 million people who lived their whole lives within the culture will be much more useful in my intellectual and personal development than reading a few novels from a tiny body of literature that does not have any real cultural impact.
With that being said, bear in mind that if you learn Esperanto, you will be able to start appreciating the content available in it in just a few months, whereas you will need many years to reach the same level of proficiency in a language like Russian. Most people who decide to learn Russian give up before they can actually reap the benefits, whereas most people who decide to learn Esperanto reach proficiency so fast they pretty much don’t have enough time to give up.
Esperanto is probably not the best candidate for a lingua franca
Note: This section explains why Esperanto is not the best candidate for a “universal” language, but it probably should not influence your decision whether to learn it or not, unless introducing a truly international lingua franca is your main goal.
Esperanto was designed to be an international auxiliary language, i.e. a lingua franca; to play the role globally that is played by English, Russian, French, Mandarin and other languages in various contexts today. This is a utopian idea—the languages of the world are too diverse to construct a language that would be equally easy to learn for everyone.
What we would really need in Europe and the Americas is a language as easy to learn as possible for speakers of European languages, which is an achievable goal, and Esperanto is in fact based almost entirely on European vocabulary anyway. The problem is that it was created predominantly by a single person during a time when linguistics was still a very young scientific field, so it shouldn’t be surprising that it could have been constructed better than it was.
Interlingua, for example, is an auxiliary language constructed better than Esperanto in terms of intelligibility of its vocabulary to speakers of European languages—just take a look at the Wikipedia article about Interlingua written in Interlingua and the article about Esperanto written in Esperanto and see how much you can understand (but both languages take about the same amount of effort to learn; Esperanto just looks more complex to the untrained eye).
I believe that immediate intelligibility to those who are reluctant to learn a foreign language is an important part of a lingua franca. If we were to adopt a new official lingua franca in Europe (which, let’s be honest, is not going to happen), I would vote for creating a wholly new language with the methods of modern computational linguistics to optimize intelligibility and learning time, unless Esperanto gains such a large following that the sheer number of speakers outweighs the benefits of a better-constructed language.
Esperanto requires a few special characters
I know this is a small thing, but it can still be a bit irritating. Esperanto uses several letters with diacritical marks that do not exist in other languages: ĉ, ĝ, ĵ, ŝ, and ŭ (and infrequently also the ugly letter ĥ, but fortunately the words that use it are quite rare).
They are used to represent additional sounds that do not have their own letters of the alphabet: ĉ is pronounced as “ch” in “chest”, ĝ as “g” in “gist”, ĵ as “g” in “massage”, and ŝ as “sh” in “ship”. The letter ŭ is used to indicate that the “u” is part of a diphthong (for example, aŭ is pronounced as “ow” in “cow”, whereas “au” would be two syllables, “ah-oo”).
There are two problems with those letters. First and foremost, they are not present on a normal keyboard, so in order to type Esperanto properly, you will have to use an Esperanto keyboard layout (or a special international one that contains various diacritical marks). While this is true for any language (if you want to type German, you will need a German keyboard layout), it is an unnecessary hindrance (Esperanto could have been easily designed without those characters).
The second problem I see is that these letters scare off (or rather put off) potential new speakers of the language. They make the language look even weirder at first sight (it reminded me of Romanian when I first saw it). I believe it would have been better to base Esperanto’s orthography on major European languages whose orthography is relatively phonemic, such as Italian and Spanish.
Conclusion
At this point in time, learning Esperanto is a nice pastime. It can be fun, it can be educational, and it can allow you to meet new people, but it is nowhere near its original goal of becoming an international language (which is something many Esperanto speakers still believe could happen). If you need to learn a language for practical reasons or job opportunities or just want to have access to a vast body of literature, Esperanto is not a good choice.
Nevertheless, don’t believe people who say learning it is a waste of time. First, it will help you learn other languages, if you decide to do that later. Second, it is the most widely spoken constructed language, so if the idea of constructed languages intrigues you, why not give it a try? Third, it is by far the easiest living language to learn and hence a great stepping stone to becoming a polyglot.
And finally, even though it is just a hobby, there are other much more unusual hobbies than learning Esperanto, and no one says, for instance, that growing bonsai is a waste of time if you enjoy it, right?
