Not to add fuel to the fire, but I have yet to see a phone or tablet work as well as a navigation system as a decent built in one. I've been in cars using iPhones, iPads and Android phones with their baked in turn by turn or apps such as TomTom, but they don't come close to the accuracy of the system I used on a 2003 BMW 5 series.
From what I gather, most phones etc suck at accuracy as they are working with a dampened signal, whereas built-in GPS units have not only dedicated antennas, but also use information from the ABS (et al) to measure distance travelled and therefore further increase accuracy.
The only advantage a phone has is that it can be mounted on the dash above the steering wheel in case ones vehicle isn't fitted with a HUD. Or, as in my case, on the bars of my bicycle. But the routing is terrible through some of the built up parts of Munich, so it's more of a general guide than a real turn by turn.