Pandora sucks nowadays. You only get 40 hours a month for free and then you have to pay. That takes me about a week to eat up. Grooveshark allows you to search for any song and also includes rare songs that people have uploaded. It seems like I listen to a fairly different variety everyday even when I put similar bands in at the beginning of my queue. The only thing I don't like about it is, once it starts playing a band it plays it a bunch unless you say you don't like it. If you give a song a smiley face, it basically plays that band all day every other song.
Overall I think it is better than Pandora since its still free, never stops due to being idle, and it has no audio ads.
Basically...fuck Pandora.