mophie air
I have been using the mophie air for about four weeks and it is an excellent backup for battery juice. I switch it on when my 3GS gets down to about 60%and it recharges it in about 1/2 an hour while still being totally functional. It seems to give me almost double my battery life and I never have to take the phone out of the mophie case since they both charge when I plug the mophie into the charger at night. Also, the size addition to the iphone is minimal - still smaller that the old BB Storm I had. And the mophie does not interfere with any of th size buttons or other iphone functions. It also looks sleek and shiny so it does not detract to the appearance of the iphone, IMHO. The only negative is that it does not fit in a standard iphone holster. I use an old BB bold case and with a little stretching at the top, it works. Given the mophie's small size, I think it is a great way to add battery life without creating a bulky iphone.