I think I had actually good results with it. Both with my kids who did improved a lot after starting to use it - their grades from language in school were going down before it.

And for myself too. I did had improvements I never got with self learning before. For me the gamification is the big thing that actually helps to keep on doing. One large reason for why I would not be to improve in language without duolingo is that makes the boring parts actually fun and keeps you engage to keep going. It wont make you fluent on itself, but it makes learning language feel less like horrible chore.