Rosetta Stone has been around for a number of years, and I've tried it out briefly, but never wanted to spend the money ($179 just for level one!) without knowing if it would be worthwhile. But Duolingo, the app for iPhone and Android, is free. I recently started using it and have been impressed with it for a number of reasons. As a linguist and unrepentant polyglot, here's my take on its strengths and weaknesses.
Babies are really, really good at learning languages, and they don't do it the way high school students do (i.e. with lots of focus on learning rules, and especially how to read and write). Duolingo does away with this tired old paradigm and treats us more or less like babies who can read, forcing our brains to learn grammatical rules without them being explained to us. So, for example, in my Swedish course I know that to say "a dog" you say "en hund" but to say "an egg" you say "ett ägg". There was no mention of grammatical gender, least of all "masculine" and "feminine" nouns, which tend to just make everyone very confused about how inanimate objects can have sex (they don't). The explicit explanation of underlying grammatical rules is what we in the linguistics biz call metalinguistic knowledge, which is nice to have if you're a linguist, but fairly useless if you're trying to learn to speak. Your smart brain is hard-wired to learn language, and is pretty good at remembering that for some nouns, the word for "a/an" is "en" and for others it's "ett". It's the way we're built (though it does require a lot of repetition; see below). And if you do want an explicit explanation, there are ways to get to it, but it's not front and center as with most language courses.
Lots of exposure to native pronunciation.
Native speakers build Duolingo, and you get a lot of exposure to their pronunciation, which is a lot better than your high school French teacher's probably was (speaking from experience here, no offense Mrs. Lacey). There's just no beating a native speaker, even if we might never be able to mimic exactly how they sound.
It fits right in with your phone-addicted lifestyle.
I admit it: I am addicted to my iPhone (and I semi-hate myself for it). But Duolingo fits in nicely with the modern lifestyle: it's on your phone, which you are on constantly anyway, and doesn't require you to go to a class or take out a book or go out of your way. It's right there in your pocket, and you can do a unit in just a few minutes while waiting for the train. They've even programmed it with some game-like incentives (lose hearts if you make a mistake! earn "lingots" if you pass a unit!) to tap into your unhealthy Candy Crush addiction. The great thing about this is that it's easy to do a little every day, which is key to learning a new language -- you have to reinforce what you've learned or you will forget it very quickly. By making a smartphone-based platform, the designers of Duolingo have made it easier than ever to help motivated learners achieve their goals.
Not enough speaking practice.
I can't speak for the whole of Duolingo, which currently offers some 13 language courses for English speakers and many more for non-English speakers (with many more in development). But I have completed significant amounts of their courses in French, Spanish, Italian, Portuguese, German, and Swedish, as well as a bit of Dutch and Turkish. So far, only the German and Spanish courses have actually asked me to speak into the phone so my pronunciation can be judged. I don't know if this means that some courses lack a speaking portion or not, but it's been inconsistent. And even when it does ask you to pronounce a sentence, it can't give you feedback when your pronunciation is bad. It just asks you to keep repeating the phrase, which is super annoying.
Duolingo essentially relies on just a few core methods for teaching you a language skill (say, a set of vocabulary words like colors or numbers, or a grammatical concept like the past tense):
All in all, I am super jazzed about this app. It's been great for practicing the languages that I already know, and has been pretty decent for starting to learn ones that I didn't. Though I really wish I had a real live Swedish teacher to explain to me WHY the 'sk' combination in Swedish is sometimes pronounced like an 'f'. But I guess I could just Google it. Brave new world, I love you.