It was pretty useful for learning kana, but beyond that I don’t think it was all that… It introduces grammatical concepts really early, and does a poor job of explaining them (は vs が confusion, for example, and doesn’t explain anywhere that there are kun’yomi and on’yomi readings of kanji, which leads to all sorts of questions about why two phrases have the same character sounding completely different). I think Duo is fine as a supplementary resource/study material, but it is toted as being the new-age Rosetta Stone (in that it’s the go-to language app), when I frankly think LingoDeer does Japanese a lot better (as it is much more focused on teaching vocabulary, but still has the issue of teaching grammar concepts too early). I kind of have an axe to grind with both apps, since I’m convinced those style of apps (and maybe just phone apps in general) aren’t really optimal ways to study languages (at least for me, if it works for you, fantastic, go nuts). My Duo Plus subscription runs out this month, and I’m not paying for another year. I have a lifetime LingoDeer sub, so maybe I’ll revisit that at some point, but again I just feel as if my time is being wasted on those apps and I end up fooling myself into thinking I understand learned concepts more than I actually do when using them.
Dunno, I guess those apps are overall fine, but I’d recommend just buying a Genki textbook instead of paying for a premium subscription though, if you’re looking for another study resource. I stand by the statement that WK + a textbook is the optimal self-study method for beginners, and it kind of sucks that there are probably tons of people who use Duo (or -like apps) as their introduction to the language, and end up getting confused/frustrated, and quitting.
tldr; not a fan, but it’s useful for specific things. I feel as if it makes you feel accomplishment without actually learning, and is a poor overall introduction to language-learning for that reason.
–I should mention, Duo apparently updated their Japanese language course a bunch since I last used it, I have no idea if it’s improved or not. Seems like they brought it closer to their other course’s overall level of features (for example, adding Stories), but I’m still not convinced. I wanted to make a note, since I remembered that they had sent out an email talking about how they updated the course not too long ago. Again, go nuts, give it a shot and see if it seems useful to you, but it wasn’t really for me when I was working through it a couple years ago.–