My wife and I recently (Oct, 2013) spent three nights in Venice, then rented a car and drove around Italy without having advance plans for route or lodging. Our only constraint was the date and time of our return flight from Rome eight days later. This type of trip is not for everyone, but I wanted to report my good experiences with Google Maps on an iPhone 5 and the Verizon data plan. By signing up in the US you get 100 MB of data for $25, and when you use it up you are charged another $25. Since roaming data in Europe is something like 20 Euros per megabyte, I had a hard time believing that this offer from Verizon was not 'too good to be true.'
Now you shouldn't be playing music or downloading videos or photos to the phone (or uploading them either). Those are data-intensive operations and you'll eat up the megabytes in a hurry.
I did consider getting a Garmin with the Europe software and researched it extensively, but finally decided to just give the iPhone a try.
We used the phone for email, text messages ($0.50 to send one, $0.05 to receive one), checking the Rick Steves site :-), searching for B&B's in our next city, calling the B&B's to make reservations, and Google Maps turn-by-turn routing. We drove between 4 and 6 hours per day, visiting Ravenna, Ferrara, Firenze, then a long drive to Positano, back to Pompeii and finally to Rome where we dropped the car at the airport and took the Leonardo train into the city for two nights.
Google Maps was running the entire time we were driving, but each unit of 100MB of data lasted us between 2 and 3 days. My total data bill upon returning was $100, less than the amount that was picked from my pocket on our last night in Rome. :-( I cancelled the roaming data service the day we got back.
Even more miraculous was the accuracy and reliability of the Google Maps routing and spoken turn-by-turn directions. Even on the back roads of Tuscany reception was fine, and we were never mis-routed. One B&B was in downtown Ferrara where the center is a ZTL, zone of limited traffic. Google did not know about this but the owner warned us in advance so by giving the name of the B&B to the policewoman we were able to drive through. It took us to our B&B in Positano, which we visited based on hearing about it right on this site (thank you whoever made that post!) If it can navigate Positano, it can do anything. We brought paper maps and looked at them only to figure out where to stop for lunch.
One more story. My wife likes her bagels. On our last night in Rome I found mention of a new restaurant called the Bagel Bar on the web, so I asked Google Maps how to get there by train. Not only did it know which subway line to take and where to get off, it knew when the next streetcar would arrive, at which stop to get off and how to walk the last few blocks. Mmmm - dinner was a toasted bagel, cream cheese and lox. And of course a glass of red wine, a first with bagels for me. (But I'll stick to coffee with bagels now that I'm home.)
So I hope our experience is helpful and says thank you in a small way for all the help this site and community provided us.