I just checked my battery and it has about 85% charge remaining after being in 4G all day with phone calls and email updates every 15-30 mins on 5 email accounts depending on the account, photos and texting. The amazing part is that I've had it off the charger for more than 7 hours!!
I'm pretty happy with my 3500 mAh Seidio extended battery and my Thunderbolt. I'd highly recommend this battery to anyone who wants to go all day without worries about a flat battery.
The battery cover antenna works perfectly too.
Sent from my HTC Thunderbolt
CORRECTION - The battery is 3200mAh, not 3500mAh - my apologies for this error.