I keep mine at default. Too high and you can't control your shots -- if you do reflex shots, you'll often find yourself still aiming while your shooting. Too slow, and you can't turn around or aim fast enough. These two problems are mainly due to the 360 joystick being wonky. If playing this on the PC, that'd be a different story, as accuracy is much more efficient, and I'd bump the sensitivity up a notch or two.
If anything, if you're playing it on the 360, one notch above default should be sufficient, at least I think, but really, the sensitivity is fine at default.
The only thing I really changed on the settings was the brightness. I toned up the brightness, until the "NOT VISIBLE" box could be seen a little. It doesn't really make much of a difference in terms of visuals, but just a few notches and you can actually see your surroundings on the Makin map. For the other maps on Castle/Roundhouse, I couldn't see much of a difference in brightness when I toned it up, not sure why, but it's working for me.