Didn't this feature already exist and then disappear? I seem to remember being able to select a roughly 20 mile square area to download to my phone but the data and the feature disappeared after either an Android update or a Maps update.
This is quite good for my android phone because it seems to try to connect to every metro wifi out there and doesn't fall back gracefully and quickly enough for it to be seamless. The iPhone has a better UX when it comes to this but still would be quite good especially when I go to out of the city and get edge or poor 3G connection, it also will work great when roaming and I won't have to worry about closing the navigation app or pressing anything that will trigger a new search when I'm out of the hotel and don't have wifi.
That said, the ability to download maps for offline viewing was supported years ago, then removed, then added back (hidden in a hard-to-find corner of the app).
My other complaint was that the area it allowed you to save offline was too small to cover a trip to another city. So you'd have to save separate areas all along the route, and try to make sure they all overlapped a bit.
Hoping that this allows larger saved areas and starting or changing navigation destinations while offline.