So...with my new Montana bought yesterday...I've now installed BaseCamp on my Mac. I'm a long time user of MapSource. BaseCamp is very cool how you can search for other people's routes online.
I always had trouble transferring files between an older 2610 and MapSource. The Windows PC would blue screen almost every time. The interface between the Mac and Montana seems to be reliable, so far.
After 10 years of using Garmin GPS units, I'm finally in a good place.