Me and 8-9 co-workers have recently started playing ARMA2 twice a week now. We played CoD4 for about two years but after we all boycotted MW2 we decided to drop CoD altogether.
We rented a dedicated server and have been having a blast running co-op missions. The game is definitely a HUGE learning curve, and is definitely rough around the edges, but there's no other game that can provide as realistic a military experience. There's very little more satisfying than working night after a night on a particularly challenging mission, then finally getting the teamwork to click and beating it.
The funniest story we had this week was that we were doing a mission that required retrieving a briefcase from a heavily fortified town, then making it to the LZ to get picked up by a helicopter for evacuation. We fought our way into town, retrieved the briefcase, then called the chopper in.
We make it back to the LZ and wait.....and wait. We can see the chopper pilot on the radar but he's moving agonizingly slow. After about ten minutes of sitting around, one of our guys goes, "Dammit, where the hell is he?" Another guy goes, "Umm....I think he's standing by our truck."
We turn around and sure enough the AI chopper pilot is just standing there with us. Apparently his chopper got shot down but he managed to eject. Then, as a good little soldier obeying his orders, he faithfully walked halfway across the map to rendezvous with us. One of my co-workers looks at him and goes, "You stupid son of a bitch!" and just shoots him in the face. We failed the mission, but the sound of all of us falling out of our chairs laughing more than made up for it.