Here's some "high quality" (heh heh) anecdata for you: I navigated from my house in Somerville to a restaurant in the Seaport district of Boston last night, in the POURING rain using public transit and walking.
Google maps literally was leading me around in circles downtown once I got off the train, so I switched to Apple Maps and it was straight shooting from there on in.
I think GMaps is more susceptible to the tall buildings fouling the GPS. Not sure why?