It’s not something I personally use, I’ve played around a little though. The reason I don’t use it is I’ve got mapping for all of France and Switzerland and parts of the UK so I don’t have high quality maps I want to us.
What I do use, from time to time, is the Garmin Birdseye imagery, these are satellite images you can stick on the handheld, they’re rightly criticised in some areas as being awful but in other places it’s not so bad. In some places the right general shape of valleys is enough for navigation and it’s better than the 1:100k or more maps the USSR knocked up to drive their tanks by.
If you are going to load maps I believe the tool you want to be looking at is this : http://www.the-thorns.org.uk/mapping but it’s an area that’s developing quickly so it may have moved on since I looked. The best place, which you may have found by now, is the garmin oregon wiki here : http://garminoregon.wikispaces.com/Custom+Maps
