I haven't tried any alternatives really, but so far for me I'd say decent. I put on an album, and once over it'll play similarish stuff. If I don't like a song I'll skip it and it seems to incorporate that feedback.

Only thing is that it doesn't seem to be too adventurous and it adheres rather strictly to the local context. Meaning, if I played a stoner rock track, it'll continue suggesting stoner rock and not much else, even though I have quite varied music favorited in my library.

Overall though I've found a lot of new bands I enjoy that way so, positive experience for me.

edit: as an example, here are the two most recent ones it suggested where I ended up buying the albums on Bandcamp. Both have quite few monthly listeners (1-2k), so not what I'd call mainstream.

SUIR https://open.spotify.com/artist/6zOeQ2hyNfqi9UMHtyTSlF

Mount Hush https://open.spotify.com/artist/13clfeXxTPsDsqzSlLIBZJ

Having said that, starting about a year ago (maybe ~1.5 years?) Spotify started inserting obviously paid promotion tracks into my auto-generated "Daily Mix n" playlists and it seriously bothered me. When my playlists are made up of very specific genres of EDM and suddenly a pop song plays from a famous person I get seriously angry.

It hasn't happened in many months though so maybe they learned their lesson. I was so mad I seriously considered ending my Premium subscription right then and there when that track played.