Here's a good review of this lens by a pro wildlife photographer, explaining why he thinks it's worth the money:
http://www.andyrouse.co.uk/index.php?page_id=174
Basically, it's a specialized lens intended for people who make their living doing sports and wildlife photography, for whom the ability to go from 200-400 f/4 to 280-560 f/5.6 in a fraction of a second, without pulling the lens off to add an extender or picking up a backup body, can mean the difference between getting a shot that might return them hundreds or even thousands of dollars, and not getting the shot. If you're in the very rarefied category of photographers who make your living on this type of work, a lens like this will probably pay for itself in a fairly short period of time. If not, then it's not for you. I'm certainly not in this category so it's not even on my radar. But that doesn't mean it's not a great lens well worth the money for some photographers.
Secondarily, the fact that the extender is custom designed and fitted to the lens means virtually no IQ loss with the extender in place, which is generally not true stand-alone of extenders, especially when coupled with zooms. Adding an extender to a zoom, even professional quality zooms, usually results in at least some IQ loss.
Effectively, it's a 200 - 560 f/4 - 5.6, and so should be compared to other lenses with this focal and aperture range, not a 200-400 f/4. Alternately, you could compare it to what it would cost to carry two bodies, one with a 200-400 f/4 mounted, and one with a 280-560 f/5.6 mounted. But here you need to consider the advantages the one lens/one body setup has in terms of weight, flexibility, and speed of response.
I'm also sure it will eventually come down in price somewhat. But it's always going to be a very top-end, take-out-a-second-mortgage lens.
