The post above is wrong. You don't need cellular for the entire trip. If you know you are headed to a no cell zone you can download the map before you leave. Otherwise, Google Maps and WAZE will keep the route ahead downloaded to your phone and you will have no idea when you run in or out of cell service.
You don't need a stand alone GPS (especially a built-in which is a pain to keep up to date). Plug in your phone and you will have the most up-to-date information for routing plug minute by minute traffic updates and POI information the GPS units can't touch. Google and WAZE will even keep up with temp road closures due to construction which would take a year to get updated in a GPS system.