I'd also add that while I have rarely had problems with it -- only came up with one USB-powered analog audio mixer that had less-than-amazing power circuitry and bled noise from dirty power being provided by USB through into the audio signal, and where I put it on a dedicated charger -- USB power can be stupendously dirty. I was watching some guy with an oscilloscope investigate various devices, and all those sensitive devices are accepting all kinds of craziness in terms of power. I'm surprised that USB power sources aren't required to provide some hard guarantees on what they can do in terms of load and response.