I bought Rock Band yesterday for the XBox360. This game rulz!!!!
We played until the XBox shut itself down sometime after midnight. I wish I knew more of the songs, I am sure that will change as more become unlocked. According to the game site and news reports, groups will be making songs available for download (The Who: Who's Next is supposed to be the first album) for a fee. I have heard Metallica will be releasing a brand new song via the game (I am sure other channels also).
I really enjoyed playing the Drums and Singing, playing the guitar is much like Guitar Hero.
It was determined our band name was not 'Classy' enough to be shown on XBox live. I bet they have 'hate' in a list of banned band names.