Like others have said, it depends on the genre of the game. For normal FPS games, I stick with normal settings (besides increasing the sensitivity). But for any flying games or mech games (like Mech Assault) I must go inverted.
I alwaays play with inverted controls. my brother calls it perverted because he plays with "normal" controls.
i suppose i think of it as a rifle with a pivot on the bracing hand, and the trigger hand as the control. in that sense, the control hand would move down to make the gun aim up.
Inverted every time. That includes vertical camera, any type of aiming and controlling an aircraft. If there is an option for inverting vertical movement, most of the time I choose that option.