I have downloaded the U.S. maps along with the maps for France and Canada. I searched for the addresses for some cousins in France and found that the street names had to be exactly right to get a hit.
I have also used the navigation driving around San Francisco a bit. It worked fine, but some of the routes were not what I consider to be optimal. It recalculates quickly and figured out my route. It looks to be quite useful, but you probably should have a car power adapter for a long trip.