I used Duolingo for a couple of languages, got pretty far in Italian and tried some German. I found it either too slow, or too beginner-ish; I didn't feel like I was learning much, and it didn't help me read news websites / novels much. While I do like the approach - specifically, the use of words in context, and the variety in exercises - it didn't really do it for me (Severin, if you're reading this - sorry, man!)
Duolingo starts at the beginning. If you think a skill is too easy, try testing out of it to get to a higher level or advance faster.
Duolingo says they don't currently aim too teach beyond A2/B1, so you may already be past that.
Having learnt a few languages with and without duolingo, I think you will usually need 2000+ words in a language, and close to 90% of the grammar to read native texts well. Duolingo can get you quite close to, but not past that mark.
