As someone said before, yes, you can download the maps in advance. However even with the map data Maemo Mapper needs Internet connection to receive route instructions. This means that if you have downlaoded map data and route info and your route is blocked for whatever route, you cannot simply take anoter route and ask MM for new route. Similary, if you miss your exit on a highway, MM won't find a new route automatically. With MM you'll always need Internet connectivity to find a route. Finally, MM reads route instructions from downloaded XML-file using speech synthesis. Usually this means that route information is usually like "turn left to Foobar street and go 47 km" instead of "take second lane after 200 metres and turn left".

Personally I like MaemoMapper, but it's horrible memory hog. I've practically given up trying to use MM and Browser at the same time. I only need to scroll the map a bit and MM has consumed most of free RAM and likely some of swap as well. On the other hand using Maps draws more power.

If I needed GPS routing, I'd choose Maps over MaemoMapper. Personally I use both.