I have been shooting with the Pentax for about two years.
I carry the camera, a 35mm, 75mm, 120mm, 80-160mm and 400mm- with grads, filters, 7 batteries and a wimberly sidekick in a lowpro dryzone.
I love shooting with this camera, and the results are really outstanding. The camera has operated flawlessly, without one single problem.
Everything is simple and easy to operate, there are some really nice functions as well, the lenses are very affordable and there are many focal lengths available.
I was in the same position as you, needing to do larger and larger prints, and looking to go to a phase one system.
I think I'm into the Pentax system for about $18,000. (I bought a couple of other lenses that I really don't use.)
For me it was a question of budget, and I don't think I could have put together my system for any where close to this going the Phase route...
Nothing I have seen or read shows that the phase has an real advantage in regards to image quality.
And I would bet the Pentax is a bit more nimble and robust.
As anther person posted once you pass thirty seconds in long exposure mode the camera builds a black screen shot and does an in camera noise reduction for the exposure, this alone is worth having the camera for if you are into long exposure works, the images come out super clean!
Built in electronic level, mirror lock up dial right on the body, big bright; really bright viewfinder! and super LCD/monitor
I really like the photographic experience, this camera just gets out of the way. right times can be a little slow for sure, but that's rarely that big a deal for me.
Its a very solid system, and a camera that you can continue to push.
It also seems Pentax will support and expand on this system, although the new lenses are not nearly as affordable.
For me I considered I would shoot this camera forever, I'm that happy with it; but alas I am hearing rumors of a 645D two!
At any rate that's my 2 cents.