My one charges off the mains in it's own little stand thing: I think it's the official 360 one, and is much better than having a wire stuck to the controller IMHO. Also means I can have two packs sitting there on Perma-Charge whilst using the other two pack in the pads
Honestly I don't know as I only used the USB kit once, maybe twice before I saw the stand on display and impulse purchased. Either way charges a pack quicker than you can use it though, I think. The stand definitely does.