I rather enjoy Play Music. Anything that I want to listen to is there, it has Pandora type radio stations, and you can download anything you want. It's no different than downloading music to an SDCard.
I'm being forced to Google Music...It's very limited relative to much more advanced players. I can't sort by decade. I can't select multiple genres without Google bastardizing genres. Unless I upload a playlist from a different player I might as well be sawing a hammer.
No ratings, no filters. It's literally a bare-bones player when technology offers so much more.