Just bought one based on a reviewer named Patric Michael on Amazon.com. He gives a lengthy review, but it's the following part that got me:
Electronically, the case delivers pretty much as promised, so long as you explicitly follow the manufacturer's admittedly sparse instructions. Unu presents this case as a "recharger," rather than an ongoing power supply, meaning that they recommend turning the device on only after the phones internal battery is depleted. Used in that fashion, the case consistently recharges the phone battery from 20% to 100% every time, though the "power low" warning light starts flashing at around 85% or so.
However, it occurred to me to go against the manufacturer's recommendations and use the case as an alternative power source, thereby saving the iPhone's internal battery from the inevitable charge cycle failure. After all, $50.00 is a lot less expensive than what Apple charges to replace a failed phone battery.
Thus, I charge the case up each night and it lasts through most of the next day, powering the phone though an "average" day of voice calls, network and cellular surfing, playing music, any number of apps being run, and perhaps a few YouTube videos. By the end of the day, the low power warning is either blinking or given up altogether, but by then I am home and the phone is more or less docked or corded for the night. Remember that this kind of usage is *opposite* the manufacturer's suggested application and your mileage may vary, not to mention probably being contrary to it's warranty. For me, I have no problem with that because a year after having purchased the iPhone 4, it's internal battery is still capable of delivering a charge time comparable to when it was brand new.