Ever since I've had an iPhone with GPS I've become totally dependent on it. It's so useful to have it in a place like New York. When I'm walking down the street I can look at the screen and the little blue dot tells me where I am. The problem is that when I'm using the GPS radio, the batteries drain pretty quickly. So I have to conserve my use. But now that I have a Mophie Juice Pack Plus -- a $99 rechargeable external battery case for my iPhone (the manufacturer sent me an evaluation unit) -- I can stare at my iPhone the entire time I'm in public without having to worry that the battery will lose its charge before I get back to my hotel.
The Mophie adds some bulk and weight to the phone, but not much. It's got a power switch so that when the iPhone's battery is starting to die I can switch over to the Mophie's battery. It's kind of like those old cars that had two gas tanks in them -- when one tank ran out you turned a valve to start getting gasoline from the other tank.
When the case is on you use the included micro USB port to charge the phone's battery, the Mophie's battery, and to synchronize the phone with iTunes. The Mophie comes with a four-LED status indicator to let you know how much charge remains.
The manufacturer claims that the Mophie is rechargeable for over 500 cycles, and that partial cycles aren't counted as full cycles. That means you can charge the Mophie's battery 10% of the way 10 times before counts as a full cycle. The Mophie's battery is designed to drain before the iPhone's battery drains, so that means when the juice pack has done its thing you can remove it and your iPhone will be fully charged.
I haven't measured how much more time the Mophie gives me. Here are the manufacturer's specs:
Up to 8 add'l hours talk time on 3G; 16 hours 2G
Up to 7 add'l hours internet on 3G; 11 hours WiFi
Up to 44 add'l hrs audio playback
Up to 11 add'l hrs video playback
This means I can play Fruit Ninja the whole time I'm on a flight from Los Angeles to New York, and still have enough juice to find my way to Forbidden Planet in Manhattan.