Yesterday at the Manchester Nikon Roadshow I was able to use and handle the 180-400 mounted on a D5. Unfortunately although it was tripod mounted it may as well have hung from a piece of string, the tripod head was as unstable as you can imagine. However, I did get the feel of it, one thing sticks in my mind was the smooth way the TC switches in and out, with a right finger, while maintaining a full grip of the camera. It can be locked in the on or off position.
I can't speak for the sharpness, a distant road sign which defined clearly with another D5 and a 600mm FL lens, was almost unreadable, even allowing for the floppy mount, so I can only conclude the lens and body AF systems were mis matched.