When I had a slow 256 MB CF card, it didn't matter whether my reader was usb or fw, but I recently bought a SanDisk Ultra II 1 GB CF card for by Rebel and decided to try shooting raw.
It was very slow to transfer pictures over usb, so I bought a SanDisk Firewire CF card reader. It is much faster than my usb 2.0 card reader.
Switching to a FW CF reader also freed up one of my usb ports allowing me to keep my my scanner and my ups connected to my mac all the time.
--ranga