I play non-inverted and can't stand inverted. The only time I use inverted y-axis is with any flying game, obviously.
I don't understand why so many people use inverted! A friend of mine who uses inverted had said a long time ago, something about leaning back and forth with the inverted mouse movements. Like if you pull the mouse back (down), you want to sort of lean back with it, so as your aim in a FPS game moves up, you kind of follow it up.
I guess that sort of makes sense, but I don't bounce around, well not too often anyway, while gaming. I think he's sitting too damn close to the monitor!
Actually the only thing that bothers me about people playing inverted is when they play some game(s) on my computer and set to inverted, next time I play the game, GD inverted! One of my buddies did this a ton to me. Happens with console too.