I wouldn't use our smartphone for navigation when driving as the screen is too small. And our portable Garmin that we sometime use when kayaking or hiking is also too small and old (it's more for topo maps).
Instead when driving we use Google Maps on our tablets (Verizon in the states, Rogers in Canada). The tablets also act as wifi hotspots for our computers and smartphones, and have cheaper cellular data plans than the phones (they also allow us to read ebooks from the Toronto Public Library :)). We haven't really noticed any increase in data usage when travelling, prolly 'cause we use a lot of data normally, but I'm sure it's there.
Google apparently does have a newly implemented feature that allows you to download all the maps for a destination beforehand, in case you get out of cell tower range. But we haven't used that yet.