I have ordered RAVPower 5600 mAh external battery and have used it for half a year.It works very great now,and it can charge my iphone4,ipad,psp,also can charge my brother's Motorola Droid.
In terms of physical size and weight, I'd place this at about the same weight as a cell phone and a bit larger than a couple of 9-volt batteries stacked on top of each other. You can easily wrap a hand around it, or slip it into a pocket, and the weight is enough that you'll know it's there.
The main thing you should care about with a product like this is how much of a charge you can get from it. My cell phone is the iphone4, and has an extended battery installed. This pack was able to charge it from around 25% to 100% over an hour or so. It did that twice, and charged my brother's Motorola Droid once. Then, it provided maybe 20% of a charge to a 'New iPad' (a.k.a. an iPad 3). For the size, weight, and price, I was pretty impressed.
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It's a nice looking product, at least to me. It feels solidly built, and has performed well in half year of ownership.
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