Additionally to not supporting off-line maps, Google Navigation has one more vivid problem. It requires data connection to run.
If, at the moment, when it doesn't have data connection, I switch from Navigation to any other application (for example to WiFi settings to see hot spots around, or any other, for that matter) - I can't return to Google Navigation back anymore until it gets its data connection. The Navigation screen is locked (it's dark) and I can't do anything with it until it finally gets its data from outside (or, I guess, sends my data into an opposite direction)...
So, I always have to remember do not switch to other apps while I'm driving (and I hope that no one will call me when I drive too.). Otherwise, if there is no data connection, I loose the Navigation... Not a good thing to discover on the road, when you may need it the most.