I use my nav system almost constantly, even on well known routes. I get a near-exact predicted arrival time, so it helps me plan a little better. And, if there's a traffic jam or construction in an area (which the unit also displays from a traffic update signal) and I want to find my own detour, I can see the alternatives at a glance.

This was my first-ever nav unit (other than in rentals), but I'd never want to do without again. There's talk that future units will actually show the speed limit of the street that you're currently on--THAT would be great.