Some of the most annoying, that's for sure. I uninstalled it when it started to uglify the icon on my desktop when I neglect daily exercise. I mean, it's totally fine if it's just me, but since it was an impulsive action on my part, I would be curious to see if I wasn't one of the many. I wouldn't be fond of my company's PR department if they lose customers over stupid jokes.

That said, after spending too much time on DuoLingo, I should have dropped it anyway. First off, one should be honest to himself and admit that it is a game, not a language study material. Which is ok, but still, I would really like to have an app that is a bit less of a game and a little more of a interactive textbook (I don't know one). Second, honestly, most of the course materials are surprisingly low quality. They kept adding all these gimmicks, animations, icon uglifying, etc., yet the core content was barely worked on. After a couple of years you start really wondering what are they spending money on. I mean, literally, do they even have paid staff working on some less popular languages, or it's just community?