The GPS logs are in GPX file format, which is XML based but also easy to parse from whatever your favorite scripting language is.
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.
Oh, that's great that you have a tool. I should have said that my reply was for the eTrex *X series that store the maps and logs on an SD card and that things might be different for different models.
