I've used it for French and German (and some maintenance practice in Russian). For German it was really the only thing I used and it got me to high A1/low A2. I was able to have decently complex conversations while traveling where German was our only shared language, so it can work to a point.

I think you realistically need to supplement it with other input/practice but I've found it really good for two things:

1) Just doing _something_ every day. Unless you go to DLI or Middlebury, you're just not going to learn a language in a month even if you do nothing but study it. If you're not living in your target language then it's easy to just not use it at all. Duo at least makes you light up the neurons for your target language every day.

2) For French I've found it's good at covering random blank spots that I didn't realize I had. My wife is french so I mostly learn from conversing with her and I'm at like B2/C1(on a good day). But she's not going to correct me every time I misuse a tense or preposition. So _most_ days on Duo I mostly get everything correct and learn like 1 or 2 new words, but sometimes it covers some grammatical nuance that I wouldn't have thought to go improve.