Scott, two great thing about GPX files is that they are xml, and that they can contain multiple tracks, waypoints etc within the one file.
What really sucks is the lack of editing software for the Mac. Unfortunately I have to use my work windows laptop to run mapsource so I can load the GPX file and then select which track I want to save.
Be careful with saving your logs on the gps itself, most garmins will reduce the resolution of the track (generalise the points) when you save the track. It's much better to pull off the active log which retains the original resolution. As a test save the active log and then pull off both the saved log and the active and compare them.
