I had a quick test yesterday with a Phase One DF vs my old Mamiya 645AFD2.
To jump to conclusions:
Focus speed a bit faster (not miles faster).
Focus accuracy about the same (although the old camera was pretty good in this regard).
Shutter release a lot better (like night and day better) than the "spongy" button of the 645 AFD2 (talking about the awkward delay from depressing the button to the actual exposure).
A lot better handling and a nicer finishing, a step away from the plastic feeling from the past.
Would I buy one? I don't know, the price is a bit on the heavy side (4200 euros list price), hard to compare with ebay prices of tons and tons of AFD2's, at 500 euros.
Plus, I shot 90% of my jobs on a view camera, but I guess your mileage does vary.
On the other hand I considered the AFD2 a big PITA whenever I need to shoot with it (mainly because of that ugly,
spongy, release shutter).
The DF 645 is a lot better, closer to a 35mm DSLR speed, at least in studio under flash lighting.