I'm a mostly single player gamer. I am not a fan of online multiplayer what so ever, due to the fact that I like working with team work than anything else. I do will play my fair share with friends I know well, though, from time to time.
Now online co-op I will do, as long as the person is a good team player. (a good example of this is RE5. I barely do online co-op with random people because mostly none of them are team players, always wanting to run ahead when I actually want to gather items, ect) But with friends? Hit me up. I love co-op gaming and I will play it at anytime.
I do play multiplayer only games though, like Team Fortress, Halo Reach maybe a few rounds of living dead, and Gears as well. I never play Modern Warfare multiplayer though. Neverrrr.
When I owned Aliens Vs. Predator, I did like playing that multiplayer. Aliens Colonial marines has me excited for it's multiplayer as well. I love the whole aspect of Human vs. Alien.