The mobile app is pretty decent, but it's not well designed. It's easy to get lost in the confusion mishmash of navigation elements. For example, if I'm playing a song, I tend to hit the hamburger menu in the upper-right corner to find the "add to playlist" function, when it's in the tiny "..." icon next to the pause button.
I'm reasonably happy with the desktop app, although it's still as terrible at organizing your collection of music as it's always been. For example, if you save a single song from an album, that album shows up in your "Albums" view, which makes no sense. And if you go to that album, it's actually a weird, special virtual album and not the real album (there's a "View full album" link to get to that one).
I was using the "Save" function for quite a while until I met the 10,000 song limit. Which is a ridiculously low limit for any lover of music, especially when album counts toward that limit — "saving" an album is actually just saving the individual tracks. You can't raise this limit; you're done, or you have to go back and unsave music.
The "Local files" function is also pretty obviously an afterthought. You can only play them to a device if your device is on the same network as your desktop machine; this also goes for syncing. To play your "local files" on a device, you have to mark them for offline play on the device while the desktop Spotify is running. How ridiculous is that?
A final gripe: After 10 years, Spotify still doesn't categorize classical music by composer and performer separately. Apple Music does this correctly.