Very true with the tall buildings. I commute to, and stay in, Manhattan every week and the Nav is the worst in there. Thank god I know where I'm going. Since street navigation-oriented gps systems all have to do a little fudging to try to figure out where you really are, my position jumps all over the place, sometimes even sitting at a red light. My car marker will jump a block ahead or back, or suddenly change from an east-west street to a block or two up, but now on a north-south avenue.. forget about trying to follow directions to a destination. I really feel for people who NEED the gps in the city, cuz it ain't happenin'.
The same happened with my Garmin, too, so its not just ours. Surroundings have a lot to do with it.