I have a Garmin nüvi 1350T with the freebie Traffic Receiver, so it does a nice job of letting us know of traffic delays on the route ahead of us, giving me the option to pick a different route around the delay. Usually, the delay is less than the detour so we just go through the original route. Google shows congestion along major roads, but it isn't as specific as the Garmin.
Using Google Maps on the iPhone isn't very good because it isn't set up to follow along with the vehicle's location. That said, however, when we had to locate a store that had relocated due to a fire and I wasn't sure which way to go, Google Maps pointed us in the right direction, plus brought us back to the car when we were finished shopping.
Finally, using the dedicated GPS has a fixed annual cost (updating the maps) instead of eating into my limited Data Plan.