I just updated my Garmin StreetPilot C530 from v 3.30 to 5.7. The Garmin <beta> Web Updater worked flawlessly.

You must be a bit patient ... it took about 20-30 seconds for the StreetPilot icon to show up on my mac's desktop via the USB connection. Once that happened, the Garmin Web Updater located my GPS unit easily and updated the software quickly. It even gave me the option to update other software on my unit, if desired. The License Agreement text even appeared (after a very short delay), for those interested in reading it before updating. ;-)

BTW: I am using Mac OS X 10.4.9 (PPC).