USB
is sloooow
is crap
wastes massive amounts of cpu power
Above comments apply to 3 Ghz Pentium 4 as well as .05Ghz 68060.
You should never use USB for high speed data transfers; that is exactly what USB is NOT made for.
USB is ok for, say, a digital camera that you connect once per day and transfer some pics over.
The worst possible use for USB that I can think of is to use it as a hard drive interface.