The problem isn't struggling with the GPS(at least not in the sense of "Oh noes, the UI is just too hard!). The question is whether or not the GPS UI is distracting the driver's attention enough to make them especially vulnerable to doing stupid(which in a car means dangerous) things.
Yeah... but if not for looking at my GPS, plus listening to it's voice prompts, plus hitting the traffic map button, plus texting from my iPhone, and using the camera to snap a picture of the traffic to send to my buddy to explain why I'm late and using my bluetooth speakerphone, and drinking my Starbucks and eating that Big Mac... what else am I going to do while in the car?
Economics is extremely useful as a form of employment for economists. -- John Kenneth Galbraith