I thought it might be useful to post something on my experience of trying to move from using Mapsource on Windows to a Mac (now have both iMac and Macbook Pro Retina both running OSX 10.8.3) using an old Garmin Quest (European version Mk 1) - the one with more memory (234Mb or thereabouts)
It's been a big learning curve and a pain, nothing seems straight forward with Garmin software.
The reason for the change, apart from a PC to Mac was that I like the Quest, I use it on my bike (motor), it might be slow to recalc, but I can live with that, all part of the riding adventure, it's small, handheld and waterproof, plus I already have it and don't like spending money unnecessarily.
First thing, I've not been able to convert my maps (City Navigator Europe v8, v9 or Europe 2008) and since they were quite old now, though mostly still good out in the sticks wanted to load something new - Garmin don't do new maps for the Quest 1 as it's not NT compatible. I've loaded maps from the Open Street Map project - http://garmin.openstreetmap.nl/ and I'm very grateful to those running this site, really useful information and help, also for those in the UK another useful link here http://talkytoaster.info/ukmaps.htm. This was quite straightforward, but only after doing some reading up on the subject and finally giving it a go! For the Quest, maps are downloaded to the Mac first, then transferred later. Something else I'd not realised, being a Mapsource user previously, was that Mapsource builds an index of the maps and transfers maps to the Quest with ability to select the maps you want in the programme; Basecamp (and Roadtrip) work differently, with Garmin Mapinstall needed to select the tiles you want and transfer the maps - important for Quest users with the limited memory. Works nicely though and gives a running update on the space left on the device.
The initial difficulty encountered was that the Quest was not recognised by the Mac when plugging into the USB post, but I loaded the Garmin Communicator plug (worked with Safari 6.0.4, but unit not found when I tried it in Chrome 26.0.1410.65) in and it (eventually) found the Quest, the trick seems to be to have the Quast turned on and running before launching Basecamp, though Roadtrip appears to find it OK.
I'm still learning to use Basecamp as it doesn't seem to work in the same way that Mapsource did, but then it took a while to get used to that. Anyway, it is possible to use a Garmin Quest with Mac, just requires some patience. So, working for now, I'll try and report back on any problems I encounter, other that those whilst learning to use the software!