Some (all?) DV camcorders transfer the raw DV data over Firewire but recompress the video when tranferring over USB. DV camcorders are really meant to be used with Firewire.
I need to capture some early HD footage from a lot of older miniDV tapes and this camera outputs via Firewire. I would really rather just buy an adapter to bridge between the Firewire and the USB port ...