The built-in updating mechanism has never worked for me - it helpfully indicates that the update "is estimated to take 3 hour(s), 38 minute(s) to download on a 56k modem". A 56k modem, for a piece of software bought in 2009, whose About box indicates a copyright of 2008.
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.
I lucked out on GPS though - in some cases, the GPS signal has died just when I need it most. Darn Nexus Ones.