Me and a mate got pretty addicted to zombies at one point and got pretty good, we got to level 27 with 3 rayguns and just held out upstairs by the grenade spawn point!! Don't often see above level 15 anymore though! The new maps should be a laugh.
Wasn't a big fan at the beginning, but it IS an amazing game, play it too much. Some of my mates get into the 30's i think on Zombies
Should start a ITR clan!!