I've never really been one to get in on the whole online multiplayer thing. PC users have been hep to this for ages, but I have a personal abhorrence for using computers as game machines (long story), so over the years I've just sort of chalked this up as 'not my deal' and moved on. No Doom or Quake or Unreal multiplayer. No World of Warcraft or Everquest. I dabbled in Alien Front Online wth my Dreamcast (complete with voice mic) and that was fun... but after the Dreamcast dropped off the planet I just couldn't muster the resolve to keep at online play. I enjoy single-player campaigns just fine, and my own preference for competitive play was definitely informed by my arcade days of live, face-to-face opponents.
But I've finally taken the plunge with Warhammer 40,000 Space Marine on the Xbox 360. As I understand it the multiplayer is kind of no-frills, and pretty difficult for its type since the weapons are so deadly. But it is pretty fun and is definitely teaching me why online frag-fests are so popular.... though I don't see myself putting making a huge time sink out of it. Exterminatus just got released last week as free DLC. It probably should've been included with the main game release, but I'm not complaining.
Exterminatus is multiplayer co-op. Basically the survival or 'horde mode' of other titles. You and up to three other players make a stand against waves of Orks, Gretchin and other baddies thrown at you with an unique selection of maps. So far I've mostly played this against my own kid. Being local our connection is good and our headset chatter fun. He's really enjoying it, though Sonic Generations recent release made it kind of a difficult job getting him back to this 'old' game!