I love Rock Band so much and can't wait until I'm able to afford Rock Band II for Wii!
I've only played Rock Band for the 360, but I'm in love with the experience. Of course you can play it alone, but the real fun is in getting a bunch of your friends together to join in the ride. Sometimes people have a real hard time loosening up and getting into it, but once they do it's so much fun. There's such a fun selection of songs, and I really like rotating instruments so it really keeps you on your toes.
Though it certainly has its competitive elements, Rock Band seems to focus more on the cooperative aspect of game play. While I enjoy putting noobs in their place, I'd much rather do co-op when I'm with friends, and Rock Band definitely provides that!