i have LastFM, Pandora, Dizzler, and last but not least DroidLive. on LastFM, just make sure to put it in high quality mode or it will sound like total crap.
Slacker has good pre-defined stations, LastFM has good tag/interest based music, including lots of international stuff I didn't see on Slacker... never tried Pandora or Droid Live, but probably worth checking out...
Last FM is without a doubt my favorite. If you enable scrobbling on your other players (itunes, winamp, ipod, etc) you have an amazing stats program that is quite telling. This is my profile telescreen?s Music Profile ? Users at Last.fm and no I don't listen to Taking Back Sunday that much. Tt's the wife's I swear.
I also love the ability to listen to stuff from my own stash of music without having to transfer it to the phone and the similar artists formula is brilliant.
Playing through the car radio sucked at first until I found the high quality setting and now I use it all the time.
Ohh also the new service at Mog.com absolutely rules. It has already replaced my Last Fm for home usage and when the app comes out next year, I will probably only use this service. It already has 3 or 4 times more songs than pandora and just opened last week.
i'm not a fan of the last.fm app. it doesn't regulate the volume between songs so constantly have to adjust. plus i feel it takes longer to buffer than pandora, although pandora is not without its problems.