Wow, very addictive. I didn't realize the XBLA had so many games in it already - I played the demo of the first, then figured why not just pick up the second when it hits. I'm pretty glad I did.
I'm very suprised there's no online co-op - I'm hoping it's added in a patch or something like that. I'm guessing with the amount of stuff on screen at one time, it'd be difficult to track so many different objects at once, and have it coordinated between two or more people.
If you want to compare scores, I just hauled ass through Pacifism - my gamertag is twishart.
Also, I'm reading some reviews of 'braid'. Looks to be pretty good - very old school platformer style, with some interesting twists.