The nav is probably accurate given the database it has to work with. Don't know how to make the data better, though. I have a 3 week old 550 and so far the nav is mostly accurate. Even Garmin has some gaps, I always bring backup in the way of common sense and a paper map in case it's needed.
It isn't broken yet but I can fix that!
2013 550i, BMW F800ST, ML350