

Going from dark screen to bright screen is almost immediate, but the Mac will take a while to go from bright screen to dark screen except on opening the clamshell or waking from sleep.

The thing is, the Mac is designed to cater to the user, because supposedly a human being knows what it wants better than a little $.005 photo sensor, so the buttons on the keyboard ( F1 and F2) override the ambient light settings for a set period of time, and then still until the light conditions change drastically enough to cause the auto-adjust to kick in again. The ambient light sensor turns the display brighter when its bright outside and dim when it's dark inside. Theoretically it should, but it is all relative.
