I bought a Garmin navigator for my car. Basic map which covers all Europe is ok, but can not be detailed (even if all streets are there). Topographic map of Finland from Garmin would have cost something around 200€ One internet shop had customer comment "why pay 200€ when you can get this free?". Lo and behold, somebody had made a Garmin compatible 1:20000 topo map of Finland from the government mapping data which has now been in public domain for several years. It works and looks much more interesting that the bland basic map which comes with the gadget. It has contours, lake depths, vegetation, everything.
The only problem is the amount of data, navigator processor does not have enough muscle to show a large area like more than 10 km across.