Unfortunately, GARMIN, like many manufacturers, doesn't do much for MAC users!. What they will do for updates, is take your credit card # and put a $50 deposit on it, ship you a proprietary memory chip with the new software, and credit your deposit when you return the chip. The chip just needs to be inserted in one of the memory slots and will automatically update your unit when you power it up. It usually doesn't take more than a minute or two, depending on the cmplexity of the upgrade.
Be aware that there are upgrades to the main unit as well as the GPS-17 antenna/sensor. They are no where near as frequent for the antenna, from what I have seen, but they do occur.
Great unit, I have mine for over 3 years!