Thats a bit like asking how longs a piece of string BB
There are so many about now,I should think they are all ok for the quality of scanned stuff dependant on which software its using.
I have a personal dislike of USB powered ones,so would recommend that you look at mains powered but connected by USB.
No doubt others have used USB connected/powered with no problems.
Are there any USB 2 ones out yet?
USB 2 is backwards compatable to USB 1 so it would be ok using either on a USB 2 PC