I use Google Music as my primary player...at this point, as my everything for music.
I also mark music for offline playback. For example, all songs that I "thumbs up" automatically get downloaded to the device (my tablet too) so I have quite a long playlist that is always available, even offline. You can make and mark other playlists to be stored offline too.
I just prefer to work with just Google Music rather than dealing with copying music to the phone, managing it on the phone, using a separate player on the phone AND still using Google Music for music I added to my virtual library that I don't have files for or for music I happened to have not copied to the phone but suddenly want to listen to. Whew.... did that make sense?
I do wish Google Music had a more robust player though... would like a little more flexibility and better equalizer functions. Guess you can't have it all.