Duolingo suddenly told me that today is, it turns out, the 7th duoversary of my registration. Seven years ago I came to Duo to learn some German and got hooked for about two years — I don't remember exactly anymore (same as German, ha-ha-ha).
It was interesting and fun, and here's an old post about it that I just dug up. Some of it did stick in my head: I deal with German-language websites almost every day, and I still recognize familiar words.
But at some point I stopped practicing. And recently the interest came back in the form of a desire to learn Chinese. A pity, of course, that such desires show up more often the less time you have to act on them.
But while I still have a little time, I'm holding on and practicing. And the app is good, yes — since it stuck with me for so long.
By the way, if anyone's interested, besides various languages Duo has added chess, math and music theory courses. I already know how to play chess, I've had enough calculus for a lifetime, but music is something I could try.