This is easily one of the best accessories I've found for my iPhone in the last three years. The case itself is very comfortable in the hand. It doesn't add much weight or bulk to the phone at all. It has soft edges around the case so it's nice and comfortable when held against your ear. (Some other cases I have used had extremely sharp edges around the face of the phone.)
I love that the device will actually recharge my phone's battery or the phone and device batteries can be used together. I typically use the power button on the back to shut off the battery extender until my phone drops to a critical level and then activate the battery pack. When recharging, only one cable is needed instead of two separate cables.
The battery pack uses a Micro USB port to recharge. With the phone in the battery pack you can also recharge the phone with the same cable. Not only is the Micro USB port more common (almost all modern cell phones, such as Blackberry and Android, use this port). Most of the Micro USB cables I have found are considerably more robust than the iPod sync cables and they are cheaper to replace.
I mainly bought this battery pack for traveling. Now, I don't have to worry about my battery going dead because I spent too much time listening to music while at the airport. If I want to watch a video, I can now do so without concern that the battery will run out before the video ends. It's GREAT!
This device should shut the iPhone detractors up in a hurry when they whine about not being able to change batteries or swap the battery out for one with a longer charge.
I had to come back and add something else about this battery pack that I just discovered by accident. You can actually sync the phone, with the battery attached, using the included micro USB cable.