I brought this battery pack for my iPhone 4 for when I'm on longer bicycle rides (8 hours plus) and I'm using GPS the entire time. I didn't need anything that gave me a full charge, but something that was light, easy to carry and gave me a few extra hours. This product does exactly that.
It is 100% compatible with the iPhone 4 (couldn't find anything that says it was prior to me purchasing it). From a fully charged battery pack and a dead iPhone 4, it charges it to 55% before the battery is drained. Running a GPS program and listening to music that gives me another 3-4 hours of life, which is exactly what I wanted.
If you are looking for something that will give you a quick charge, then I would highly recommend this product. For the low cost it can't be beat. However, if you are looking for a battery pack that gives you a COMPLETE charge (depending on the device) you may consider something with more mAh (Milliamp Hours).
It comes with a USB cable for charging the battery pack and takes about 2 hours to fully charge from completely drained when plugged into my computer's USB port. It has a small on/off switch on the side and an easy to ready LED light on the front. Green is on and charged, Amber is on and drained, no LED is off.
If you have a device that can be charged by plugging it into your computer, then this battery pack should work on that device. Instead of plugging the device into the computer, you plug it into this device and turn it on.
Just to further test its capabilities, I was also able to partially charge a Kindle (2nd Gen), an iPhone 3GS, iPod Nano (3rd gen), and a BlackBerry 9700.