I have a pretty extensive iTunes library, so I would never pay for streaming. I'll have to try out Slacker. I use Pandora a lot. It works for what I need it for. The audio quality is decent. It has a nice size library, but it seems to play the same 7 or 8 bands per band selected.
Example: I listen to Them Crooked Vultures radio a lot. It plays TCV and the band member's other bands: Nirvana, Foo Fighters, Led Zep and Queens of the Stone Age. Then it throws in The White Stripes, The Black Keys and the Raconteurs. That's as many bands as it seems to play.