Holy crap, Rock Band 2 is awesome. I've been playing it, both solo as well as with my two daughters, and it's been a whole lot of fun. It's awesome, covered in awesome sauce. In addition to all of its merits as a solo game, the fact that I can play it with two kids, one of whom is only 6, and still have good results says a great deal about its family-friendliness.
I'll now be telling everybody I know to upgrade. So far, that means ... um ... you.