I am a Mac user and all of my experience is with the Garmin Vista and only OSX.
Software:
I find that you need Virtual PC (I use the XP Pro version) to flash the firmware of your GPS. And to run the Mapsource software for uploading/downloading maps, routes and waypoints.
Mac GPS is great for converting waypoints dowmloaded here on Geocaching.com and uploading them to the GPS.
Hardware:
You need a USB to serial converter. This is where it got tricky for me.
Keyspan has one that works for the firmware flashing and for uploading waypoints. Still fails sometimes and you have to do it twice.
I can also get it to work for fairly small maps. Anything larger than 2 MB's or so have failed for me. Also stressing the processor (by playing iTunes, for instance) seems to help keep the failure from happening as soon.
A company called IOGear makes a converter that uploads the maps/waypoints without fail but doesn't seem to work for the firmware flashing.
So with both together I can do what I want but it was hard getting there.
Hope this helps,
Lance