As soon as I first saw them mentioned on Boing Boing back in September, I knew that I'd have to at least try some of the new USBCELL batteries from Moixa. I bought a pair this week; here are some initial impressions.
Essentially, these are cut-down AA size NiMH rechargeable cells with a flip-off top concealing a standard USB plug and internal charging circuitry. You plug them into a spare USB port, they charge. A charge light comes on to tell you something is happening: they start blinking when they are 90% done, the light goes off when they are cooked. This takes about 5 hours from empty: an overnight charge plugged into your laptop, for example.
As about 30% of the length of the cell is taken up with the USB plug and its cover (held on by two little elastic cords), the capacity is much lower than modern NiMH cells at around 1300mAH. By comparison, a standard cell (at perhaps 20% of the price) might be 2100mAH these days. The cells are a little fatter than standard, so although they worked fine in my Garmin GPS they won't fit in a Maglite AA torch.
So if cost and capacity are so much worse than a standard cell, what's the attraction? If you're travelling and are already carrying a laptop but need a couple of rechargeables for some small device like a GPS, you can just take these and leave the NiMH charger and the power adapters at home.