My brother got a Xbox 360 with CoD2 and PGR3. The reflections in PGR3 are some of the best ive ever seen. CoD2 is a lot less technically impressive, but it runs at a constant 60 FPS, unlike the poor PC port (even on high end computers).
I won't repeat everything Ricoder said, so ill give you my experiences with Xbox Live. My bro started by playing CoD2. We got into the game relatively quickly (despite the painful 30min registration for the MS Passport acct, entirely on the controller). Thats where the positives end. After 15 clearly consequtive events, we deducted that the enemy was INVISIBLE. On one section, the enemy was crouching right in front of him, stood up right in front of his face and got melee'ed in the back of the head. In another, his teammate shot at the doorway and an enemy corpse flew out of nowhere. If this didn't happen, we were greeted with unplayable lag or random disconnects. Needless to say, we never completed a game out of like 15
PGR3 ran flawlessly on Live though. Until they iron out the bugs and have a better game selection, I would hold back on buying it.