Well, I bought the HTC extended battery and did 2 tests with it. I did 2 10 hour tests with it using WiFi (even though I'm in a 4G area) and saw ~20% drain both times with medium usage (60+ text messages, browsing tweetcaster pro, checking weather, several 10 min calls, a picture or two +emailing). Considering that I can get 24 hours (100%-20%) on my stock battery with light to medium usage, I've decided to pack up the extended and use it for hiking and traveling only.
The battery is huge. It makes the phone (for me, a 5'11 180 pound guy, with large hands) awkward to handle and hold. It is really heavy and uncomfortably so for any calls that pass the 10 min marker. That said, I'm impressed with the battery time that I've seen and it isn't THAT big but too big for me.
After putting the stock battery in ... I can say that I'm relieved to feel the phone with 1/2 the weight and it is much easier to hold in one hand, feels much much better. I gladly sacrifice the extra battery time for having chargers all over work, car, and house with a lighter/thinner phone. I also would have a hard time taking my extended battery in public as I already have people asking what phone I have with stock battery (they're impressed by it).
My opinion is, if they were going to put an extended battery on it, make the entire back the same width instead of having a huge hump in the middle. But whatever.
I just wish taking off the back of this phone didn't require ripping a plastic cover off. The Droid 1 had it right with the slide off battery cover (and I had 3-4 batteries for that).
Senseless Das BAMF 1.7 with CF 2.0 kernel (both by Adrynalyne) with super low voltage (my settings). 245mhz-1Ghz.