iPhone or standalone GPS are not super reliable for walking in the city, especially Tokyo with tall buildings and narrow side streets, as GPS requires line of sight to the sky and mutiple satellites are required for correct positioning.

If you are walking along wide boulevards or crossing parking lots or public parks, then yes it's excellent.

I used google maps (with a local data plan) and MapFan (Japanese standalone GPS app that requires no data) trying to access certain addresses and neither of them is very good. It typically lacks behind by 500m or so.

At least with google maps you can use street view to see the actual neighbourhood or things along the route.