It's much more user-friendly than either of those other two, as Ian said it's properly tagged and arranged.
It also only includes music that they have licences for, unlike Grooveshark who let people upload things and then look at whether they have licences, so it's less questionable legally.
It's also a higher quality of sound than on Grooveshark, although I haven't heard Pandora.
Just when Spotify started sucking then.
What sucks about it? Mine is still amazing.