A couple of my friends and I are investigating which battery cases to get for our iPhones. I have an iPhone 3G and they have the iPhone 3GS. The cases should work with either. So, I went googling around and here is what I found:
The conclusion seems to be, if you’re wanting to maximize bang for your buck, Accessory Power’s EnerG Slim-Fit seems to be the way to go. It has a 2400mAh battery and costs just $39.99 at Amazon. Keep in mind the cover exposes your iPhone 3G/3GS and only protects the rear. If you want a battery case that is fully covered and with a lot of juice but don’t mind bulk, KONNET’s PowerKZ seems to be the right choice.