A male great slaty woodpecker (Mulleripicus pulverulentus) in flight. I have just bought one of Nikon's new high frame rate bodies and was having a play, you can now find me anywhere in the forest - just listen for my Nikon machine gun. This type of shot is one use I have come up with for it - so far, much easier with high frame rate than with the D800.
Its the big one - its not really typical of me as I usually buy my bodies second hand (I screw them up quickly with forest and sweat). I was in the market for a second hand D4s but people were asking silly prices so in a moment of financial madness I bought the D5. Now I have to convince myself it was money well spent (its going to be hard).
12fps is worth the money when shooting action. Even action as small as a creature drinking and catching that exact moment where the last drop of water hangs on the tongue. Catching such a shot only works with high frame rate.
For shooting a 600mm lens from a blind, which you do, I think it's a great investment.
After the Nikon F5, I have never owned a big, professional body, but if it is built like the F5, surely it will last longer than most of the other models.