The basemap is just a fixed routable map that is used as a fallback when detailed mapping isn't available. If you have detailed routable maps (CityNavigator or CitySelect) downloaded in the unit covering the areas that you visit it will work great. The only difference that you'll notice is when zooming out to get an overview (which means the unit has to show the basemap as detailed mapping would be to cluttered), the screen will not have as much information as in Europe.