Google Maps does not require cell service either. You can download the map before you leave and have the entire day's journey setup. However, you do not get live traffic which I find to be the MOST valuable option. No GPS has it without a separate cellular connection and even then I find it to be a pitiful amount of information compared to Google Maps.
Even when you do use data, the amount of data used is very minimal. I measured less then 300MB of data used on a 350 mile trip.