To me, it's basically Rock Band with the addition of Battle of the Bands.
Being released for Wii, 360, Ps3, and Ps2.
Also, the drums look nicer.
Also, also, you can semi-customize the songs.
Also, also, also, I plan on getting it.
IMO: I think it's gonna be better than Rock Band, mainly because since Rock Band was made first, they can take it and add to it what was wanted by consumers to make it their own. Better customization on characters, better drums, customizable songs, hopefully better equipment, etc.
Also, also, also, also, a picture of the drumset:

Hopefully it won't break after a couple days.