I know just about everyone has posted what they think about Halo 2, but I will anyway. 

It rocks.

New weapons and vehicles are great. Single player cut scenes are much improved and the story is more interesting.  Multi-player is the best multi-player available on the XBox. 

Speaking of XBox multiplayer, does anyone know what the architecture for XBox Live is like?  When I play a game on XBox Live what does the Live service do after I meet up with my friends and start playing?  Why is it that a game like Halo2 can handle 16 players with no problem, yet when I play Rainbow Six 3 with 8 there is lots of lag?  What kind of hardware/hosting centers does XBox Live have behind it?  I wonder if Robert can answer this.