"I turned on the numeric battery option (jailbreak settings) to better monitor the charging. Whenever I saw the iPhone come close to 100% I switched the charging off to conserve the battery. By Monday morning the battery pack had 50% battery life left according to the LED status, and my iPhone still charged over 70%... If tethering the internet, streaming, or even playing tunes on the go, the iFuzen could be your best friend."
Joe Rocha - iPhoneSpies
You can read Joe's full commentary on Auzentech's excellent i.Fuzen HP1 for iPhone4.