Having variable voltage on the USB output scares me, however the form factor and user replaceable batteries is slick.
Same here, I had a small variable voltage usb battery pack I used to charge/run my ipod years ago.
Once it accidentally got moved to a higher voltage output and it fried both the battery and the charging circuit in the ipod.
I think if I were to get one of these I would set the voltage, then open it up and permanently disable the voltage selector switch for the usb port.
Other than that, it looks like a nice battery.