If you are OK to use a mobile phone or a tablet with a Windscreen mount, the best option would be an Android phone/tab with Map Factor Navigator and Google Maps. Map Factor Navigator is free, and you can use it for most places. Google maps are pretty accurate. Or you can get the Sala maps installed from Di*log. I guess M*bitel provides it too. This way you can detach the nav system and take it with you if you are on a hike or want to read a book, or even to make a call.
EDIT:
http://www.mapfactor.com/en/ - just found out that MapFactor Navigator is available for Windows phones FOC. You can check it out yourself. They provide free offline maps (about 4MB) and provides free updates as well.