So here's how I currently use this stuff. I still use Pandora because it has superb "station" technology. Grooveshark can't seem to guess good alternative music for me based on a playlist. So what I've been doing is listening to Pandora for variety and taking the songs I really like there and finding them on Grooveshark so I can play them whenever I want. So I'm slowly building a nice little playlist for when I want to just hit play and only hear shit I really like.
Honestly, if Pandora would let me skip around, rewind, and reorder a station, I would never leave them.