I've used the TomTom software on my iPhone (use that whilst on the motorbike). Seems to work quite well. Certainly does the job & seemed to be accurate location wise (just takes 15sec to "warm up" if you've just turned Location Services on). The audio prompts are timely & accurate. In a car setting, you'd just need a phone mount either on the window or one that connects onto the airvents.
I have also used TomTom for iPhone and found it to be very reliable and easy to use. I never had an issue with an address not being found. Would recommend for people with iPhones...