I have not had any Android phone that I liked the auto-brightness setting on. My wife's Inspire works the best out of all the Android phones we have had.
On the SkyRocket the auto setting always seems to bright for me.
I like Brightness Profiles
. You can set what ever levels you want (or quickly turn on auto-brightness). I have mine set up at 4%, 8%, 12 %, 25%, 50%, 75%, and 90%. I have a shortcut on my home screen for quick access.
How much the screen brightness hurts your battery life depends on how much your screen is on, in addition to the brightness level.