I use an Mbox with Logic on my iBook 600 using the Propagamma driver from
www.usb-audio.com. I use the lowest latency figure (7 ms) all the time, be it with audio or softsynths. The whole thing kicks are and worked first time! More than can be said for the ProTools/Mbox combo. While researching this combo, I was in email contact with a fellow user who stated "the Mbox works better with Logic (with the Propagamma ASIO driver) than with ProTools". I'm not sure what Digidesign would think of that comment. When I get my CoreAudio driver, I will really be giggling.
I can't imagine there being a big difference between firewire and usb interfaces. The Mbox might be usb, but I am proof that it works, it works very well, and it works without trouble. YMMV, but if you follow the intructions it shouldn't.