This has been my experience thus far:
OEM Battery - Typical days use includes:
Wifi Tether & Pandora on Bus from one hospital to the next while at work
Tweetdeck/Email on all day, Ocassional fruit ninja during down time.
Texts/Calls to co workers in various parts of the hospital all day.
Pandora/Grooveshark on the Bus/Drive home
Usually dead as soon as I start the drive home.
Seidio 3500mah Battery
All of the above plus additional Pandora/Grooveshark, heavier data usage due to less fear of battery death, Touchdown Exchange app on all times.
I charge my battery over night on Thursday, unplugged Friday morning at 6:30 AM. It is now 2:40 PM Saturday and I still have a 40% battery charge. I have not done the calibration on battery use in CM6 yet because I'm still waiting for my phone to die.
Totally worth it IMO. Also a bit of clear scotch tape on the side of my phone keeps the battery door in place with no issues.