My experience is that mostly it's useful as a guide in tunnels, but common sense needs to be applied too. I was driving through a section in the Swiss alps last summer and at the end of the tunnel I had to immediately negotiate a junction. Because there were road works there were delays and the gps hadn't a clue where I really was in the tunnel. I used common sense and ignored the GPS other than get in the right lane in advance. OK - it's good that it can switch modes as it knows it's in a tunnel, but drivers have to use common sense as to the rest of the information (IE speed and position) on the screen.
I swear there are some out there who would have decided that despite the fact their car was stationary, the moving car on the screen was more accurate as to their position! I do know of two guys I work with who refer to the sat nav as Jane (tomtom voice) and one of them eve thanks Jane after each instruction. I think some people genuinely think the gps contains a real person sometimes.