Pandora has pre-fabricated stations now too, and they're pretty good.
A couple more thoughts:
Slacker has better control over fine-tuning the stations.
Pandora has QuickMix, which plays stuff from ALL of your stations.
Oh, and...win for both services:
http://zzj.itf-inc.com/s2
Saves your songs. It also includes a Pandora desktop client that has unlimited skips, no commercials, etc.