Much like ARMA's head tracking Thread - FaceOff Paintball mod for Half-Life 2

FaceOff has much the same head tracking functionality as the military shooter ARMA which uses TrackIR or FreeTrack. The aimpoint is decoupled from the view and you can lean around corners with your head. FaceOff's implementation has the notable additions of head roll leaning and head translation.

The roll leaning is more comfortable as you don't have to lean your whole upper body but the tilted image makes aiming harder, mouse direction movements no longer directly correspond on the screen. I would be inclined to reduce the amount of view roll. The head translation is quite effective at giving the impression of holding a weapon and allows a small amount of peaking around obstacles.

Using head tracking as a cover mechanic in a first person firefight though is quite awkward, it is unclear where the body of the player is, how much is revealed and at what point the gun will stick out from behind cover without going too far. This cannot easily be done quickly in the heat of battle.
Head tracking really shines for looking around the world but the markerless face tracking in FaceOff isn't accurate enough to do this, being marketed only as 'Realistic Camera Shake' which isn't very useful.