Gears of War is phenomenal. I just got back from playing co-op with my brother for ****ing hours. This is one of the most immersive games I've ever played.
When your playing on a 50" TV in high def with surround sound, and your running full bore in a rodeo run as a 12 foot tall monster Berserker is screaming like a banshee while it's footsteps are thundering right behind you, and your heartbeat actually starts to quicken. That's when you know that this game is awesome.
Like SaiyanPrideXIX said, this is the absolute king of console FPS, Halo has nothing on this game. It's one of the best shooters I've ever played, and the game alone is worth $500 (Canadian) for a 360. I know I'll be getting one, and I'll have zero regrets.