I rented one in Arizona a few years ago and drove it from Phoenix to the Grand Canyon. It actually got 21 mpg on the hwy, but that was with me babying it the entire way, driving 60-65 mph the entire way up I-17.
Handling? jeez... I guess if you just stepped out of a Peterbuilt dump truck, the Navigator might seem like a nice handling ride.
Even down here in 'truckville' Okla, people are dumping these monsters left and right for Priuses ...