The variety of languages available ensures that you will likely find a language you want/have to learn. The simplicity of the site and the app ensure that you will be able to learn at least the fundamentals of a language in your spare time.
The website offers an impressive variety of languages. At the time of this review, there are eleven languages that are out of beta, two still in beta, and ten languages in active development. That's just for native English speakers!
In addition to brief lessons that feature a mixture of vocal and written lessons, Duolingo features a remarkably innovative system where aspiring multilinguists can spend time translating works in order to gain a stronger grasp of their language of choice. The material consists largely of be online articles that touch on a variety of subjects, thus ensuring that the content to be translated will appeal to someone. Moreover, since this practice doubles as a business model, the website has a clean, simple interface bereft of ads or fees(there is an on-website currency called "lingots," but those are earned through achievements, not through exchange of actual money).
In addition to the very reliable and functional website is an app available for Apple, Android, and (all three) Windows Phone users! The app is somewhat more minimalist, but is also very efficiently designed, and even has limited offline functionality.
Duolingo is not without flaws.
The website, in an effort to serve as many people as possible, is largely server-controlled. While there is a very active community, there are no personal lessons and very few actual people involved. As such, the speech lessons use voice recognition that is... inconsistent at best. When translating a sentence, Duolingo is not always capable of capturing nuance, and as such will sometimes mark right answers wrong and wrong answers as "right enough."
The translation portion is, seemingly, all web content, and mostly articles. As such, there is no translating actual recordings, nor published works. When translating, it only offers the option to translate from a foreign language into your native one, and not vice-versa. In order to do that, you must change your native language setting and then sign up to "learn" your actual native language.
e.g. If I am learning Dutch, and want to try my hand at translating an article only available in English INTO Dutch, I have to tell Duolingo that my native language is Dutch and I'd like to learn English.
The app, while functional, is very limited in scope. You cannot do translations through it as of right now, the lessons are more abbreviated, the speaking exercises obviously rely on server functionality and thus require you to be online. The offline functionality only allows you to practice unlearned lessons, you cannot get a random assortment of exercises or download an old lesson you'd like to go over.
Ultimately, Duolingo is no replacement for an actual teacher. This is, at best, going to teach you a language up to the equivalent of a 200 level college class.
Be sure to supplement your education. Watch videos in the language, possibly sign up online with some service that allows you to practice communicating with a native speaker. Duolingo will NOT teach you everything you need to know, but is always a useful tool.