I use the official extended battery from Verizon, which really doesn't add all that much. It did seem to me like the battery life was poor on CM7 with GB650 until I learned about, and used, a battery calibration app. I think, especially with my having switched to the extended battery, the calibration idea is sound. It corrects the DX from mis-reporting that the battery is low when it really isn't.
When I ran the calibration, and the DX started reporting only 10% battery left (you let it run down from fully charged until the phone dies before recharging it) the DX actually ran another 3 hours past the point where it first showed 10%, and that was with the screen on for much of that time.
Unless I use the DX quite a lot more than "normal" during the day, my extended battery rarely falls below 50% now, running CM7 and GB, between nightly charges. I run the screen at auto brightness.