You really should know the streets well, and not just rely on the navigator. Once I was on the way to a chasunah in Lkwd with a guy who had one, and we were on Ave J and E15. Anybody would know to go to Ocean Pkwy and get on Belt to Verrazano. However, the guy insisted on following instructions to drive thru Boro Park to Fort Hamilton Pkwy. The amount of lights and double parked cars and blockages on the narrow streets in BP took an extra hour.
Also there is a famous story in Wall Street Journal about a guy who almost drove off a cliff as his navigator told him to continue on a certain path. At last sec, he turned around when he saw water and saved his life. You have to be careful.