Well, I've played some Team Deathmatch online, and Zombies with a couple friends.

TDM is pretty much like every other CoD TDM, which is to say you kill people with guns, or explosives, or knives. It seems to me that the gameplay has a bit of a blended feel of MW1 and MW2. I haven't tried other game modes yet - my computer is barely keeping up with the game in multiplayer, so I'm waiting for some Black Friday/Cyber Monday computer upgrades to really jump in and play a lot online.

Zombies is pretty fun and hard as hell even with 3 people. I've never made it further than round 7. The maps aren't terribly large, nor are there very many of them, so I'm hoping more will be available later. Due to the relatively small size of the maps, small quantity of maps, and high difficulty of killing the zambies, it can be a bit repetitive trying to get to the end of the same levels over and over, and even when you make it to the end, not really having much to do other than spend all your money on defenses, and then die when they're overwhelmed. Overall a neat addition, but not the mode that I'll be playing primarily.