I've done disk benckmarks and the USB 2 connected drive I have workes faster then when conected to my firewire (just awee bit faster). However, most of my USB ports are fulls, so that's what decided it for me. :)
i work everydays with a laptop and an HD and DVD burner on USB2. Really no trouble at all. The external USB 2 7200 rpm is faster than the internal Toshiba HD btw...