I bought this battery-case last friday, and it works pretty good, it has 3000mAh, which gives me 1.74 battery time (if I don’t use the phone while charging).

Reasons to have a battery pack?. Well, I don’t like to be on the bottom of the battery, call me paranoid but I just don’t like the idea of having 0 power on a device (this happens with my laptop too). And knowing the iPhone has a 1715mAh battery getting it close to discharge is quite easy.