If there's something that irritates me in majority of FPS games, it is the Middle Age leftover known as gunface. So far I know only Operation Flashpoint and Arma games that don't have this "feature", and since I played them, I won't ever describe game utilizing gunface as realistic.
Technically making muzzle converge to the aiming direction isn't that complicated as one could think. The main gameplay feature is that such solution forces people to think more about where they stick the muzzle into. This would also make them more tactically aware: IRL shooting directly over teammates head is not very wise thing to do.
I have to say that playing old Operation Flashpoint made me great paintball player mainly thanks to this "muzzle simulation" - I can always estimate direction of my muzzle with stunning precision without help of any aiming devices, unlike my friends who did not played this game