I've used one of these for a while:
http://www.westontech.com.au//show_p...tegory_id=1024
It's a BlueEye O2 USB2/firewire combo. Was about $80 when I bought it (12 months back). They've got ones with card readers & USB2/SATA combos now (seems like a good future-proofing route).
Performance wise, it seems great. USB is faster than firewire in my setup (came with both cables). Bit noisy (depending on the drive in it at the time). Pretty easy to swap out the drive.
I don't like your chances of getting a host powered one - not enough current supplied by USB to spin the motor.
I've also used Maxtor One Touch drives with clients &, gotta say, I've had trouble 2 out of 3.
Logifuse