Seriously, using USB to transfer video is like trying to push a brick through a garden hose... raw DV footage takes up a LOT of bandwidth, and USB is like a straw, wheras firewire is like a tunnel.
On top of the faster transfer rates of your video data, you also get a more responsive camera when you have to rewind/fast forward or go through something frame-by-frame. There's very little lag, unlike USB. Going Firewire is a cheap upgrade, but if you're halfway serious about video, or plan on compiling footy on a regular basis, it's a must.
These are some data rate comparisons from a USB and Firewire external hard drive (as an example):
Read Test:
* 5000 files (300 MB total) FireWire was 33% faster than USB 2.0
* 160 files (650MB total) FireWire was 70% faster than USB 2.0
Write Test:
* 5000 files (300 MB total) FireWire was 16% faster than USB 2.0
* 160 files (650MB total) FireWire was 48% faster than USB 2.0