This seems like a good small reserve battery. One nice plus is it is the smallest and lightest reserve battery I have owned. It uses a standard mini-usb connector so you can charge it through a USB port or any of the many wall adaptors you probably have.
To get an idea of the capacity of this device it provieds 1000mAh at 5 volts, a Nintendo DS Lite is 800mAh at 3.7V and a typical Motorola phone is 900mAh at 3.7V. So, one would expect the Duracell to about double the run time of any of these products (provided you have a USB charger for them). Note a typical high capacity NiMH battery is 2400mAH, at 1.2 volts. The power available in these devices in Wh or V*A.
This battery is a great size to carry with you all the time. It should provide enough additional power for that emergency phone call, or to power a PDA if you still have one.
One may want to wait for the next model; it has twice the capacity 2000mAh and a charge indicator http://www.duracell.com/us/smartpower/products_PowerReserve.asp. I expect it will probably cost more as well. If you need even more capacity today you can get a 3400mAh device http://www.amazon.com/Proporta-30423-USB-TurboCharger-3400/dp/B002DYGQI6/ref=sr_1_1?ie=UTF8&s=electronics&qid=1253240582&sr=8-1.