Duolingo's marketing of "learn a language in 5 minutes a day" or whatever their similar slogan is, is bad. Duolingo won't teach you hardly anything at all in only 5 minutes a day, and even with considerably more time (30 minutes to an hour a day), on it's own it is unlikely to teach you a language. However, in combination with other learning tools like classes, immersions, comprehensible input, etc. It is a very valuable tool. I finished the German class in about 2 years, and I found it helpful, and wished that the Duoloingo German class continued further than it did.

Yeah, I agree, I don't like aspects of the league, and I think that the way they apportion XP encourages less-than-idea ways of spending your time. Basically, if you use Duolingo exactly the way they encourage you to use it, and only that way, you won't get much out of it. But if you are self directed, recognize the ways in which it is useful, and use it as another tool alongisde the rest of your learning, it's really helpful.