Duolingo is basically the app equivalent of flashcards. It's a little smarter than that, but not much. If you're serious about learning a language, it absolutely has a place. But Duolingo __alone__ is not going to make you competent, much less fluent.

Personally, I'm happy with it. I took years of Spanish and started with Duolingo in French with zero prior French, and I can now recognize phrases and hold some basic conversation; for 5 minutes a day with no significant continuous time commitment that's not bad!