Rock Band
I've gotten up to a Hard-ish skill level on the Bass and can dabble in the other stuff. Rocking the band is really fun. The problem is, the rest of the game.
The networking blows. When an XBL-enabled account is playing, the game takes a minute or longer after every set to communicate with "Rock Central," a task that should be trivial (sending the band name and a few scores along with a small band logo image). The menu system, as I've mentioned, is retarded. And the hardware reliability has really gone down the tubes; drums are being broken now, one of the ports on the USB hub is acting up, and I think the mic may be losing sensitivity.
Of course there are easy kit replacement programs in place - and there may very well be patches on the way - but the game is doing its best to make the fun part unnecessarily inconvenient.