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I can't speak for the Power Skin...but I just took delivery of a Mophie Juice Pack Air and I must say I rather like it. Got it for cheap on eBay too.
 
I have the Powerskin for a couple months now and I have stopped using it. The case itself is fine and is a bit thinner than the Mophie. However there is one BIG flaw (not in control of Mili), when using the power charging the iPhone keeps getting a error dialog box "This device is not compatible......". It becomes a PIA, especially when I am doing something with the device such as playing music. The music stops when the dialog box is displayed.

I guess you could initiate and let it charge and when not using the device, but that's not my usage pattern.

I emailed MiLi about this issue and they responded that both they and Apple are aware of the issue and it is caused by 3.1.3. They said 3.1.2 is not affected by the case.
 
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