I have the $16.00 extended battery.
http://cgi.ebay.com/ws/eBayISAPI.dll...MEWN:IT&ih=003
I've had similar results as syanni85. It was sketchy at first, but after about a week, I'm getting phenominal battery life. Took it off the charger at 5:00 this morning, made about 10 calls, streamed about 30 minutes of video, checked e-mail 5 times, took and edited some pictures, streamed bluetooth music for about an hour, and used GPS for an hour. It's now 10:45 PM and I have 50% battery left. Awesome!