Initial Thoughts on Duolingo
I've used duolingo for a week or two now, and I think it very good. I've found that the constant translations in the lessons really force you to focus and understand all the little details of spelling, gender, number, case, etc. I've got some experience with Rosetta Stone, and I prefer Duolingo. I hope it continues to improve. The one thing I don't have a clue about are the web translations. Given my pitiful proficiency in German, I have not been able to translate most of the material as it is not matched with my skill. I suppose they will be valuable as my proficiency reaches a 'working German' level--but now they don't help anyone.
Don't expect it will be much better soon when you will advance. Duolingo has general problem with choosing sentences according to current course level. I hope that team is working on it, because now - at least for me - mastering through translating is practically non functional.