This lens and the Canon 100-400 IS, (I have both), is one of the reasons why I do not own a Nikon body. The AF of the Nikon is the over riding feature for me and I would have one in a heartbeat, but there is no equivalent of these two lenses for me, ( not to mention the Canon 70-200 F/4 ). The Nikon 80-400 seems to be optically the equal of the Canon 100-400 but even the Nikon enthusiasts are looking for an update in its autofocus performance.
The 400/5.6 is excellent value. Almost all of my bird images using my 100-400 are at the 400mm end so I use it almost exclusevely for BIF.
There is no doubt that Nikon's 200-400 F/4 is an excellent lens but it is big, expensive, and if you are at 400 for most of your bird activity the 400 F/5,6 is far and away the best value in term of cost and performance in my frugal view.