Fortunately you can pretty much ignore the Garmin software and just mount it as a USB drive to get maps on or GPS logs off.
The map format (.img) is proprietary, but has been reversed engineered, so, for example, there are tools for converting OpenStreetMap data into the format. You can download maps of many areas from cloudmade.com.
I usually download my logs onto a Linux machine, but it works just the same on the Mac (using the device as USB mass storage).
Dealing with Garmin devices is only tough for the new fitness devices...proprietary binary format :( I wrote a ruby module in C to stream parse it which I plan to opensource soonish.