I've actually been quite unimpressed with the Nav unit so far. For long trips its fancy and fun but not entirely useful. Its nice to know the distance remaining and if there's a gas station nearby, but those have never stopped me from driving anywhere (I've driven coast to coast several times and to the Great Lakes and back from Texas). The Navteq gets really confused in places like downtown Dallas and Houston and is worse than reading street signs (read that: it will get you lost even when you have some idea where you're going, I'm minded of the stories of European drivers finding themselves emmersed in rivers for having followed it).
The only benefit I see to it is the one I purchased it for: a custom mounted LCD in the vehicle.