I sit there rotating my phone sometimes to point in the direction I am facing. Last thing I need to worry about while lost is "is that turn really a right turn like it shows, or left because I'm headed south?"
It might be easier to know which direction your turn is, but using bearing vs. north=up maps makes it a lot harder to learn where things are around you in general. I keep my "next turn" vehicle-relative, but the map display itself north=up, which seems to be the best of both worlds.
I liked the existing SatNav in my Prius for this. I could split the display, the left hand side showing the 3D turn-by-turn navigation view, and the right showing a top down, north-oriented view. On longer trips, I'd also zoom the right map all the way out to show start and end, so I could visualize the entire trip.
