I have used one of those devices personally. I got mine on a sale at REI. It is bascially a small black plastic box not much bigger than two 2 AA size rechargable batteries which fit inside of it (which can be swapped out with other regararable sets enhancing its utility in my mind over the kind that use the built in battery). It has a MiniUSB port which can be used to charge it through the device either with a wall charger or car charger. It then has an output full size USB port which can be used with any standard USB cable for charging a device but also comes with a little MINI usb cable with several interchangable tips such as micro USB, miniUSB, and some of the custom Nokia tips, etc.
There is a little switch on it that swithes it from charing the battery pack itself to charging what you plug into it. There is some sort of circuitry to regulate the output voltage with a little LED to show you what mode its in.
As I mentioned before it hold two recharagle NiMh AA's that can be swapped out. Each rechargable AA is rated at about 2000-2500mAH depending on battery make. I have a Droid2 and the battery on it is rated something like 1400mAH (I have a case on it and dont really want to take it off to double check that). If you are watching it and rememebr to turn off the charging mode when its done with a charge you could probably get 2-3 charges out of it, but if you forget and leave it in charging mode after its done charging it will drain that set of rechargables overnight if you leave it on all night.
The one I have is made by Solar Focus and is called the Battery Bank. It can actually be used with a seperate solar panel you can buy to keep the battery bank charged. I just used mine with a couple of sets of AA rechargables I charged up in advance.
I got mine so I could go off the grid for a few days on a backpacking trip and still have power for my Droid 2 if there was an emergency. I was pretty impressed with Verizon's network at that point since I was in the middle of nowhere in Colorado-my buddy on Sprint did not even have coverage in the nearest town yet alone up in the mountains). I had coverage in the place we camped the first night (my buddy with Sprint had nothing) so that we could actually make calls to check in with our wives. Out on the farther trial I had enough coverage at a couple spots to get some text messages through.
