I'd cast my vote for National Geographic Topo. I've been using it with my 60CSx and it's way ahead of anything else out there for a Mac right now. It features GPX upload and download as well as interfacing very nicely with my 60CSx for track, route and waypoint upload/download. If you get the state version you'll have 1:24,000 topos of your state with a workable interface. That's much better than the 1:100,000 scale topo maps you'd be working with in the Garmin MapSource software.
Only downside is the price at about $80 per state. But if like me you do 90% of your exploring in one state I'd say it's completely worth it...at least until Garmin FINALLY comes through with OS X support.
Mark