After well over 2 years in service, my factory battery was completely incapable of powering the phone for more than a few minutes with the screen off. Turning the screen on meant a nearly instant shutdown.
The repair went incredibly well. Despite this being a particularly difficult phone to open, it can definitely be done and I made certain to watch the teardown video a few times before going in myself. To be on the safe side, I had ordered replacements for both of the removable rear panels, but was able to reuse the originals in both cases. Everything came apart and went back together exactly as shown in the video and I encountered no issues. The new battery actually seems to be a bit better than original ever was and I'm quite pleased with the results.
When prying behind the battery, be careful not to damage the LCD screen ribbon cable that runs vertically back there. The iOpener is all the heat that you need to open the device and no heat gun is required.