i had the brightness all the way up on mine, with auto brightness on, and after going into a COMPLETELY pitch black room, the thing was blinding me, i had to manually turn it down the the lowest setting in order to look at it. then, i turned the lights on in the room, and it stayed so dim i couldn't see it.
as the poster above said, it only changes if i sleep it and wake it up... sometimes.
i think apple has an issue they need to work out with it. the auto brightness feature on my MBP is SO sensitive i turn it off. just accidentally swiping your hand over the sensor while typing will dim the screen, or if someone walks behind you and blocks out the light, or if you shift your body to the side... its quite annoying to have the screen fluctuating so much.
apple, please find some middle ground!