I've completed the German course on Duolingo. It taught me a lot, definitely, but I am by no means fluent. There is quite a bit more vocabulary I could learn, as well as grammar. I think what will help with that is immersion, such as translating, reading, and writing German, or whatever language you are learning. Still, I practice on Duolingo almost every day to keep up with what I've learned, and I really do like the site.