last week i went on holiday in the UK & thought i would take the opportunity to see how Google navigation would work in car using my iphone. My car setup is no nav, visual boost, extended bluetooth & i had my iphone connected via USB.
i found google maps voice navigation works fine - the directions are sent through the car stereo which is muted when these are being read out. this is all great so long as the phone screen is not set to display the actual route map.
If you try to use maps on the phone display for visual navigation, for some reason the voice commands revert to being read out via the in-car telephone interface with a phone call to your own phone, which then takes a good 10-15 seconds to hang up each time. Also a new call is initiated every time the phone screen updates the map displayed, rendering it unusable for any practical purpose.
so if you want google to give you voice directions only, it works really well, but if you want/need to see the map as well the only option is to not have the phone connected