It is finally there and just at the right time! Now it is possible, to use Google's brilliant navigation without an active internet connection.
Offline navigation becomes crucial when you are on a trip outside of your home country. Mobile roaming plans are still too expensive and, actually, they have become obsolete nearly in any other country than Germany, since you get pretty well through the day with WiFi hotspots. But if there isn't any hotspot nearby and you got lost offside, you want to find the next subway station or the path back to the right track. The only thing, which is necessary in such cases: before you start your trip, you have to download the specific part of the map - and that's it!
I'll test the offline-ability on my current trip to Washington, DC.
Update: November 20, 2015
It works! And it's exactly what you expected: fully offline navigation! Google Maps has become an even better companion for you next trips.