Yeah that one has been for sale for the last 4 months that I know of. It comes up for a couple weeks then disappears for a couple weeks and comes back... Supporting the thing without shaking will be your biggest fight. I think the focusing mechanism on these looks interesting and have always wondered if it was faster than the standard design. Not much information out on it but here is what I have seen.
Luminous landscape
I was looking for long glass for the 645 system. I opted to go with the 645 A* 600mm 5.6 than this lens. If I had bought this lens the jump from 300mm to 800mm would have been too great and I would have needed a 400mm... If you're really interested I bet the seller would come down in price as it has seen a couple price drops already...
Edit:
Yeah, as tuco says the weight issue too especially for travel. Small planes in Africa really limit your luggage weight or rake the price up (buying two of the six seats sorta deal).