I don't know about PCs, but the USB adapter (Belkin, I think) I got for one of my G4s was terrible. The software was glitchy and weird. Worse yet, it would routinely cause the system to crash hard. Again, that experience was on a Mac.
Still, USB interfaces use the CPU. For small peripherals that is fine, but I wouldn't bog my computer down with stuff like a network adapter if I didn't have to.