I quite like Gears of War. It's nowhere near my favorite game, but it's a good alternative to Call of Duty. It plays differently, and different people like it for different reasons. I love the story to it, and I want to see how the story ends out.
The online multiplayer is far from a great feature in my eyes though. It's quite laggy, and it gets annoying when everyone just rushes with the shotgun. Plus, I think everyone hates the 'host advantage', where the host has more powerful weapons. That's just plain annoying. Horde is a fun addition though.