Its somewhat good

Just downloaded App and update firmware on both Oregon 400t and NUVI 3790 using a iMAC 27 with OS X 10.8.2 Mountain Lion. I will say GPS units need to be turned on, using correct UBS for each unit as provided at purchase from Garmin, after Garmin icon comes up open Garmin Updater located in Launchpad. HOWEVER, I was unable to download my life time maps and had to do to my IBM machine for that, 6 downloads, 4 upgrades and 10 hrs later, my maps are current. Hope this helps.

Lee J.

Dragonbears about Garmin WebUpdater