How well do the maps work when you don't have cell coverage in most cars? Just fine.
Same is true of the phone. Either you can buy any offline mapping program ranging from $10-$20, or just use what offline maps exist in Google/Apple maps (Apple maps once it starts a nav route no longer requires a network to get to where you are going).
Those are all updated automatically, for free, and I can chose the navigation I think works best for where I am.