For car navigation I have used, like most of us I am sure, both a phone based Sat-Nav and a dedicated device (Garmin). The dedicated device gets my vote every time.
At 49 GBP purely for software I believe most peoples money is best put toward some Sat-nav hardware.
Isn't Tomtom the supplier of the mapping software on the iPhone 5 debacle. Hard to believe that Apple could take a working product and screw it all up. Surely Tomtom must have played a part there???