For gamers who have yet to get into Rock Band, this is the perfect time to start. Rock Band 2 features all of the great options from the original game plus a whole new and upgraded set of modes to fulfill just about anything you want out of the experience. For Rock Band veterans, there's enough new content in the sequel that warrants a purchase in my opinion. Plus you won't be punished for having already downloaded music packs since you can bring those along too. At the very least, you're getting about 100 new songs for $60 which is still a deal in itself and you can use your existing instruments.
In the end Rock Band 2 is the definitive Rock Band experience and no gamer with a love for the genre should pass it up. Now who says that sequels can't be better than the originals!