I have been using Sprint Power Pack whenever I go out of town and recently when I had power outage at home. it is nice, but you should not use it as your main charger. It charges your lithium ion battery in one stage instead of 2 stages. Therefore your device battery life becomes significantly shorter. more info on batteries and charging at www.batteryuniversity.com

PS you can get Sprint Power Pack on ebay for $12.99 shipped, then you just need a cable for your device.