For flying airplanes and helicopters and stuff like that, I need it inverted. But for normal, walking-through-the-level FPS, I don't see how anyone could invert it. I mean, you are moving the cross-hair to shoot the enemy....up is up, down is down
I don't think of myself as moving my head in FPS games.....I think of it as moving the gun, and my gun goes up when I push my mouse forward, and my gun goes down my I pull my mouse back