Duolingo is fine for casually learning some languages, but over the 2+ years I've used the app, I've noticed that they push more and more for paid members. There are persistent bugs with some of the lessons that they never fix, the ads at the end of the lesson don't start muted just so they intentionally annoy you, there's no way to turn off the Duo nagging like 'accepting a harder challenge.' I truly appreciate Duolingo as a free service. I don't appreciate being nagged and needled to pay.