slacker had a really nice interface, although i couldn't figure out how to make a custom station...do you need to upgrade? the audio quality also seemed pretty good. no skipping in the song. however, it does not work well when it's running in the background, it immediately crashes, and the comments on the market confirm this.
Pandora runs swimmingly. I have been trying last.fm today, and not really liking it. I can't figure out how to add artists like Pandora. I'm sure it's not intended to, but it pisses me off.
I wish Pandora had last.fm's selection of music.