I came across a great find today. OSM maps on Andriod smartphones. Go to Google Playstore and search for OSMAnd and install that. Then add the world map and local countries (eg Thailand, Laos, etc). I was exploring the Laos OSM map in OSMAnd and comparing against Google Maps, and its pretty detailed.
I also followed the instructions posted for generating Garmin maps and generated a map for my old Nuvi. Not got round to loading it onto my Nuvi as I can't seem to find a Mini USB cable.
Anyhow I think the Garmin GPSs are looking old tech now, especially when you can use smartphones for a couple of thousand baht.
OSMAnd allows you to add POIs, which will get uploaded back to OSM once you are reconnected with the net. It also updates itself from time to time from the net, and its default behaviour is to allow offline usage.
There is also another Andriod app called Navfree that can use OSM maps. However this does not seem as good as OSMAnd.
So you could have your Garmin GPS and an Andriod phone as well, providing backup for each other?