I was a huge fan of Grooveshark for a long time. But then recently, once Spotify came out, I didn't want to always fire up my browser for music. Plus it was distracting when the Grooveshark tab was nearby. I guess the Spotify app is just as 'nearby' but it feels separate. Grooveshark has a really intuitive interface, has songs I can't find on Spotify and iTunes, and was great for discovering stuff with others' playlists. But at the end of the day, I want a native app for music and Spotify is filling that need for me.
Great for getting all the music you want. Since the discovery tools are just okay, you'll want to use it when you know what you want to listen to. Similar concept to Spotify, but is primarily web-based. I prefer this, but others may favor a desktop client. There are (or at least were ) desktop versions, but they have not caught on with most people. As I said with the Spotify review, Grooveshark is great for parties. Both of them are the same in my book.