I'm not a huge COD multiplayer fan. I find more times than not it's way too dominated by little kids who are insanely good and the learning curve is too steep.
But Extinction is a really fun update to the traditional 'Zombie Mode' of other COD games. Now it's all against Aliens (I know, another stereotype, but at least it's something new).
The team mechanics remind me a lot of 'Left 4 Dead' games.