Originally posted by Macario you can't, but I thought you wanted FF for it's superior IQ, well MF is even above that. If you really want superior IQ, you will go for that
The 645D isn't even a full frame 645 format, it is also a cropped one. People want full frame to be able to use the mount design of their lenses to gain superior IQ to a up to a point. Others have already mentioned the price difference.
I have always said in the past that switching to the 645D is the same as switching to a Canon/Nikon. If you have a set of FA lenses, and pentax releases a FF camera then you can still use those FA lenses (you can even use some of the DA lenses).
There is also the size, the 645D is considerably larger than the 5D2/700D. The size difference is much larger than 5d2 compared to a k5.
I see the 645D as competition for the 1d/3ds and for those it is a viable competitor. I would have been happy if they released a D700/5D2 competitor.