It is used by many games, including:
So 3ARC holds on to the IW 3.0 from COD4 and modifies it. Black Ops runs far better than MW3.
MW2 had some issues, but those had more to due with poor development by the devs. OMA and MLC. The hacking was also horrible in that game.
MW3 is on a new engine. It is separate from Black Ops in every way, so the "Lag Comp" issues are not from that game.
My question to all of you is this. What engine do you think 3ARC should use in the next title? Do you think they should build off of the IW 3 engine again, modify the IW 5.0 engine or create a new engine all together?