My brother bought Halo 3 and I tried playing it. I found the controls way too hard to use. Having never played an FPS on a console, I couldn't manage to overcome how much better using a mouse and keyboard is for controls.
I think it would have been a little easier if they let you invert the X coordinate along with the Y coordinate. The camera in 3rd person games is always controlled like that and so maybe it would be easier for some people to use that instead.