I got a chance to play it last night. Love it.

Things I learned:

You have to turn on one of the joysticks before the mic will work.

Singing the falsetto bits on "gimme shelter", while not necessary (you can sing at any octave), is incredibly fun. And tiring.

You can use the headset that came with the xbox instead of the mic if you want. Works well. Pressing A on the joystick will work as the cowbell.

Sometimes it's handclaps instead of cowbell.

The singing part is really easy on the "Medium" skill level. The game is very lenient about getting the pitch right.

The drums are incredibly difficult, even on the "Easy" skill level. It's not lenient at all about timing. You have to get the hits exactly on time, even the slightest amount of slop is not tolerated. I would often hear the thwap of my stick on the drum and see the bounce of the correct pad indicate on the screen, and it sounded and looked right, but it counted me wrong. Only when I concentrated very hard and tried as hard as possible to get the timing right would it count it as correct.