Bought Crackdown for Crackdown, the beta was just a little added perk.
I liked playing Halo2, but stopped for pretty much the same reasons Fieldkillah says. I hated playing with all the ignorant little kids. Decimate me, own me, whatever, that's cool - you don't need to go on being an arrogant little prick about it (granted, age isn't always a determining factor for this, but it really helps.)