This is one of those things that while true and useful, I personally don't realize or think of it until just after I need it. It's always frustrating to get to a destination without signal and need to plot the return trip, or try to make adjustment in the middle of a dead zone and realize you don't have maps.

I keep an atlas in my back seat just to be safe now.

The nice thing about the built-in nav is that it always works everywhere in the U.S. Google Maps only lets you download an area about half the size of Washington State at a time and you have to manually select the area to download. Fine for local use, but too tedious (and too much disk space) for a long road trip.

?? The Google Maps I use offers to download the maps if I ask for directions on a long journey. It doesn't seem to have download limits and I never need to think about it again.

Have you heard of Open Street Maps?

You can keep offline all the maps you want.