I use it for refreshing languages I already know (French and Latin from school, Spanish, German and Welsh from evening classes.) I'm currently doing Welsh evening classes, uwch2ii (around high B2, I think,) so my Welsh is quite a bit higher than Duolingo, but it is useful for learning other vocab, especially about science. I am much worse at remembering vocab compared with when I was at school.
I have also been learning Dutch from scratch, but the grammar is similar to German, as is the vocab mostly. I have bought a grammar book alongside because it's crap at teaching grammar, and I like grammar.
I did a CELTA qualification 18 months ago, and the course leader was very scathing about Duolingo, as it lacks context. I do agree - I could talk about rhinoceroses and turtles in Dutch before I could say useful things like, "my name is..." There's a reason why formal taught courses usually start with talking about your name, your family, where you live, ordering a sandwich and so on. A friend has been doing German from scratch (having done no languages since French at school,)and says not writing anything is a big drawback - though Duo has suggested writing stuff down after each lesson, to be fair.
It has its place. It's not as good as a proper course, but that won't be available to everyone in all languages.
I was told why the Welsh course is obsessed with parsnips - It's because pannas is an acronym for subjects to be wary of in language teaching -politics, and that sort of thing. I forget the details though. It's an in-joke for language teachers, apparently...