It's a fun way to kill like 15-20 minutes a day, it's probably the closest thing I waste my time on that's like a video game these days yet 100x more productive. I've got books for learning Russian and German to go through along with using Duolingo as a pretty good option for practicing basically everything else. It's fun to compare how I got Spanish down via immersion without really practicing too hard on anything outside of rosetta stone (Didn't teach me a damn thing, was a very bad tool back in the day)... Just reading a book from the 1950s (Madrigal's Magic Key to Spanish -- highly recommended) and sinking myself into that world for a while took so many years. I'm pretty sure I would have done better off with some better programs and proper study instead, but it definitely gave perspective to where tools like Duolingo have their place and where they're the most useful in pretty much giving you gameified drills and tests.