This is the first thing I configured on my new macbook air. I prefer to manually adjust the brightness for what is comfortable for me. It is easy enough to do with the keyboard shortcuts.
In safari, 50% seems nice to me. So much white, a bit dimmer is better.
Editing pictures, watching movies, playing games, 75% is better.
Outside, 100% works so much better then I thought.
At night, I'm at a few bars.