I get extremely good battery life with both my extended battery and the regular battery. I'm talking up to 2 days on the extended and at least 24 hours on the regular battery.
Impossible you say?
I'm rooted and running CleanROM 1.6, with his supplied kernal. Not overclocked or underclocked. Here is what I've done that I think gives me the best battery life:
I disable wi-fi and bluetooth and only turn them on when I need them.
I turn off 3G while at work, except for my breaks and lunch. Calls and texts still come through, but no email.
GPS is off, except when using Google Maps.
I use timerrific to automatically control screen brightness, ringer, vibrate, wifi, and bluetooth
I also have Rom Toolbox Pro and used the Auto Start Manager to disable the following:
Google Maps
Facebook-don't use it
Market
Titanium Backup
Usage Monitor
Volume+
WeatherBug Elite
Words Free
You may have other apps that automatically start, so once you have a fresh reboot, check your running apps after about 5 minutes and disable what you see fit.
I also have removed all bloat that was left in the Rom, like VZW Media Manager, Flicker and maybe a few others. Hint - First try freezing any app that you want to remove first, just in case something doesn't jive without it, i.e., apps that rely on your Contact information, might cause it to instantly FC.
I hope this info helps all of you to increase your battery's life.