Better Battery, No need for a Data connection - What happens when you get a Crap Data Connection etc.. in Google Maps??? Voice Control, Bluetooth Handsfree Calling...
I'd just get TomTom or Navigon on your phone. Both are good and with a decent Brodit dock and power lead, battery life isn't an issue.
Better Battery, No need for a Data connection - What happens when you get a Crap Data Connection etc.. in Google Maps???