i keep it on auto bright all day and it works out well.
once it gets to be dusk out i use a widget called Q Bright to lower the screen down to 20-25% or if i am at a bar or restaurant at night i also put it down to about 20% for the rest of the night. if i am in bed reading i lower it to 12% or as low as 5%.
I agree with Mr. Smith full bright ever since I bought my first smartphone. To hell with battery life. That is the same reason I buy an extended battery, so I do not have to worry about it so much. Full bright, always 4g (even when home with WiFi).
Auto brightness here..the phone seems to do a good job regulating what it needs..better than the Droid 2 I had anyway. I will say that I never left the latter on auto because unlike the Rez, I definitely noticed an increase in power consumption on the D2 if brightness would be way up.