Serious Sam is on of my favorite franchises on the Xbox and PC, and blasting through Serious Sam II on Xbox (via online co-cop) is a great way to blow through a Saturday afternoon.
The Xbox version's not going to win any graphics awards, but the visuals are colorful, bizarre, and like the dialog and overall premise, is so ridiculous and non-serious as to be a ton of fun.
The game has a lot of fun with itself, which makes it fun for me. The pace and volume of baddies is absolutely insane, but with co-op in some of the heavier moments, the frame rate does glitch a bit.
Plus, you can generally pick the game up new for $10-15 on sale ($20 MSP), and even cheaper used.