These have been around for some time.It looks like the usb plug take up about 1/4 of the battery so it would have less capacity I would think.I really can't think of any reason why I would need these.
You might find a small USB charger more useful, then you can use decent cells
I looked at them a few years back but decided they were a bit of a niche product and that the majority of people would be better off with a separate charger and higher capacity cells. Its a clever design though.
So not bad for 2 AA or 4 AAA, more than that though it'll take a while.