The current google maps caches map data, when you're on wifi it will download your surrounding area. Or if you use Maps and go to another city it will store the map data you're looking at.
I don't believe it is temporary, my maps cache is currently 38Mb, it doesn't clear itself, I have to do that manually.
I've used Google Navigator to drive from one side of the country to the other and had no problems at all, there were no dead spots, it seems to pull down data well ahead of where you currently are, it even alerts you to traffic problems further along your route.
I have a TomTom, but it doesn't get used anymore. Google navigator might not be as feature rich, but it will get you to your destination.