Here's my problem with the android GPS, and for that matter with cell phone google maps - it all works perfectly when you live in a huge country, and where the possibility that you might need to use the navigation features offered by GPS without ever leaving the country is fairly large - hence you are not really worried about the data charges, since you are using the same operator.
BUT. If like me, you live in a small European country, where within the country there is practically no need for the GPS because you know most of the country by heart. Thus the only reason for using a car GPS navigation is when you leave the country. But that's exactly the moment the huge roaming data charges begin to apply. Therefore the only practically viable option unless you don't care how much you spend on your phone costs is by using an offline solution like a Tom-tom or Garmin device.