I used to keep it on low all the time. Got tough to see in some situations so I keep it in the middle now. I tried auto but didn't like how it would constantly change if I was in certain lighting configurations where a slight shadow (or my putting my finger over the sensor) would suddenly change the brightness and/or turn on the home screen lights.
auto.
choosing a setting drives me nuts. there's nothing more frustrating than having it at its lowest setting, in bright sunlight, and then trying to figure out where on the screen will turn the brightness back up.
auto ftw.