I have exclusively used my iPhone and Waze for the last several years (disclaimer..I have an unlimited data plan) with no serious trouble. I almost appreciate the traffic data more than the directions.
Regardless of what you use, I think it most important to "scout" your trip ahead of time using your tool of choice (I typically just use Apple maps). Before any big trip I feel like I've driven it already, sometimes often. If it's a big and new route, I'll make notes and have them on hand for the copilot in case Waze, or other circumstances, through a curve during the trip.