uNu’s Ex-era Modular Battery Case for iPhone 4/4S is a pretty intriguing new charging accessory, and it has won the “most make sense” comments from professional.
This modular case ships with four main components: the main backplate, a colored frame, a flat back, and a removable battery-packed cover. The backplate and frame are designed to stay on at all times, with users swapping between the thinner back and the one equipped with the battery as desired. Part of the appeal is customization, as numerous colors are available. As with most battery cases, a Micro-USB cable is included to charge the 1700mAh cell. The fact that the battery is swappable is also a bonus.
For 80USD, not only are you getting a battery pack that performs very well, but also the convenience of a much thinner protective case when charging isn’t necessary. Admittedly, this probably makes the most sense for those carrying around a bag or purse and not just an iPhone itself.