Just a refined definition (as a Premium subscriber), you can cache whole albums, artist exclusive channels (i.e., a random playlist from any given artist), and rewinding of most songs on any channel (except news/sports).
Depending on your musical taste if you are looking for an all you can eat on-demand music service, I would recommend MOG. However, in terms of preprogrammed music stations with no commercials it's hard to beat Slacker plus sub.