The U.S. edition of the Official Xbox Magazine, Holiday '07, has gone in-depth with the Xbox 360 version of Rock Band. The verdict? A 9.5, and the title "Gaming's most intensely rewarding co-op experience". This also puts it ahead of the Guitar Hero III score of an 8.0.
Harmonix CEO Alex Rigopulous also mentions that they are considering releasing "authoring tools" so that you can put your own music into Rock Band.