I'm a heavy internet user, and my Iphone on average lasts 7 Hours then dies. No calls or texts (i have another phone for that) but i do use the internet loads. Today i used it even more than i normally would. Checking the web pretty much constantly just to really test my shiny new battery pack. I started using at 08:30 am and the phone died at about 15:45pm
I plugged the pack in, and in just over an hour i had a fully charged Iphone and bear in mind as it was charging i was STILL using it. By the time it was fully charged i had two lights left on the pack, which is about a quarter left. So i could have finished off the Iphone AGAIN with another 7 hours of heavy usage and still had an emergency 15% battery juice if i really needed it.
Add to that you can charge the pack anywhere with USB, or standard plug. It's small and thin for what it is, easily slides into the pocket and because of its USB connection, you may be able to charge other phones/devices. Although i personally have no need to do that. It's a great product and that's more than enough juice for my needs.
It's not cheap, that's why it looses a star. But that's the only reason. For heavy Iphone users it's essential. Good, now provided it doesn't break on me i'm happy.