HDR is officially awesome. I just updated to iOS 4.1 and I was really skeptical on whether or not the new technology would actually make a difference. This is a down and dirty example comparison of without and with HDR technology.
The photo doesn’t take any longer to capture, and it just takes about a second to save as an HDR version. With HDR turned on, you get two copies of the photo - one HDR and one original and you can customize this in the Settings.