The $3000 Nikon D800E versus the $10,000 Pentax 645 for landscapes... Surely the difference will be big? No?
The Pentax is said to be very good, and it's the economical end of medium format cameras (medium format has a larger sensor than the "full frame" cameras like the Nikon). I've been rather attracted to it, in abstract. But it seems that compared to the Nikon D800E the image quality gain is quite marginal, and the camera is much slower, much more expensive, has much fewer lenses (and expensive ones)...
And the D800 has, like has been the case for DSLR vs medium format always, much better low-light ability, and a much more advanced autofocus system.
I don't doubt that for many people, the D800 will be the "medium format killer".