I have been using a Quest like this for 7 years. It takes lots of patience to prepare the routes in Mapsource. Converting recorded track logs is easier than starting from scratch, but navigating in an unknown area is pretty difficult.

I have a good collection of Green Lanes and By-ways recorded and stored this way.

I have also used shareware Topo maps on a Quest in Morocco a couple of times.

I use an old Navman icn520 sat nav (wince device) unlocked and run memory map wince version on it. I have memory map v5.1.3 installed on the pc which has the option to automatically install the mobile version on to Navman. I then have over 8Gb worth of OS 25k maps for the UK (pretty much all of them) and put on the ones I need for the area. I have never really used it except a bit of green laning in my land rover. Just plotted a route on the pc version, then sent it over to the mobile. Worked just fine, but you cant navigate it with, you just view the maps and put waypoints in. I wouldnt recommend using an unlocked Navman as its a bit ropey, but I was going to invest in a cheap chinese wince device and run it on that. Would be loads better. But it works. and was ahem, not costly.

According to Garmin, the Nuvi is waterproof, rode through rain with mine as well, and still working fine

http://www.youtube.com/watch?v=ALDiGADCl9c