I am also using duo lingo to learn french, and honestly, I love it! It seems to be a very thorough language learning app. It also seems to keep you interested because it is challenging to get from one level to another, I do not leave the first level until I have attained a perfect score in each lesson.
I don't think you can learn a language exclusively through it, but it is good for any spare time and to practise. It is quite fun to use, especially compared to other methods.
I'm using it, I've been falling behind a bit lately. It's a good supplement tool if you are learning the language in class or on your own. Practicing with others is really important, learning a language without practicing conversation is difficult.