I recently posted a similar question here, trying to make sense out of Pentax's medium format strategy. In the meantime I have decided to get one in the next couple of months. Here are some of my thoughts.
For me it is one more albeit expensive item on top of a long list of existing P645 lenses and accessories. While it may be one of the most expensive items on that list, its cost is still minor compared to what else I have. The complete system with telephotos, macro, and soft focus options is so valuable that it makes sense for me to add this one.
I went into medium format at a time when MF was vastly superior over 35mm in terms of tonal values. Today, I am not so sure whether "vastly" is still true. These 40MP sensors pretty much do what I have been dreaming for decades ago. Ricoh appears to have its plans, but I'd be pretty happy if they keep on making the 645D, perhaps a wide angle and a standard zoom, but that would be all I'd need for the perfect high resolution system for myself. I am not sure whether I’d be terribly disappointed if they wouldn’t make a full-frame successor, even though it might be a nice option. Thus, either one of the 25mm options would make sense for me.
Investing into a new system is always an ordeal, since it isn’t limited to the camera or the first lens. When you go on a trip you need so much more. Then there is the different handling and different color processing. If I (ever) was to once more go back to 35mm (I have 4/3 and m4/3) I'd invest in a Canon system as their lens line-up seems to fit the 35mm philosophy even more. To me, the D800 and the P645 are so close in terms of creative value that it wouldn’t make sense for me to own both.
Having a Hy6 system and a Horseman frame, I got one of those 24mm Schneider XL lenses. There have been some discussion about this lens on the net, but mine is top notch and likely beats any retrofocus alternative, including the Pentax 25mm. My particular setup is also very usable, even without tripod. Thus, if I need fast maneuverability, the Pentax 25mm will be my choice; for best image quality the 24mm Schneider will be a nice alternative.