I use my iPhone more for photography, video (both taking video and playback), and web surfing than for talking, but I'm always running out of battery, especially when I go on vacations and day trips when I'm taking a ton of photos or trying to keep my daughter. I usually have to start shutting things down (internet, screen brightness, etc.) halfway through the day to maintain any battery.
I started looking for some sort of external battery or solar charger for my phone, but couldn't really find anything. And then the Mophie Juice Pack came along and it's PERFECT. It's an EXTREMELY lightweight case that snaps right on to the iPhone, with appropriate cut-outs for the cameras and switches. It does add a bit of thickness to the phone, but really nothing like I was expecting- it still slides easily into pockets and can be toted without any thought at all, and fits comfortably in my hand.
You charge up the Mophie case with the included power cord (plugs directly into USB port in your computer or the USB port on your iPhone cord). You can also charge your iPhone *in* the Mophie- if you plug the case in while the iPhone is inside, the Mophie will charge both.
When in use, the Mophie will use its own power first, and then when drained, it will switch to the battery in the iPhone. That way you can snap the case off and just use your iPhone on its own charge. There is a switch on the case to have put the Mophie in "standby" and use the iPhone battery first.
Overall, I'm THRILLED with this product. It's lightweight, easy to use, and for heavy iPhone users, you can just snap the case on and use your iPhone like normal. HIGHLY recommended.