I agree with jonesdavide, battery life was so-so when I first got it (11/6), but it has progressively gotten better to the point where I had no reason to charge it overnight last night. Not bad for 2 1/2 weeks. I'd like to get a spare battery and the Evo replacement sounds good, I didn't think it would fit in the G2 but it says here that it will. Good news!
12-10-2010 UPDATE: I ordered an Evo replacement battery by Seidio, taking a chance that it would fit in my G2. This is a 1750 mAh battery and I ordered the charging cradle as well. The instructions say to continue charging 2 or 3 hours AFTER the light turns green for best results. This is no problem with the charging cradle, as I typically leave the spare in for 24 hours to charge. Anyway, the new battery does seem to last a day and a half, and this is right out of the box, so I fully expect it to get even better after a few weeks. NOTE: I had to modify the battery in order to make it fit perfectly. This consisted of filing the plastic border on the bottom of the battery on each side just a bit. NO MODIFICATIONS WERE MADE TO THE PHONE. The way manufacturers seem to design the batteries to fit a particular model seems to be by changing the notch shape at the bottom of the battery frame. The G2 seems to have two plastic pins that fit into a small space in the middle of the plastic frame on the battery bottom. The Evo frame has the notches at the back of the frame, so I just filed maybe 1/32nd of an inch off each side and now it fits perfectly.
NOTE 2: this isn't an inexpensive battery, the battery and charging cradle cost 45 and 25 respectively. As the battery is made with Japanese Lithium cells, it is probably worth it. I have 2 batteries for my work-issued Blackberry, and the Japanese-made battery lasts significantly longer than the Chinese-made battery.