Neil-
I have enjoyed "playing" with one and have used a variety lenses with it. (Mamiya RZ, 645, Hassy, Sinaron) I have been using 5d I and found most of the medium format lenses sharp enough to satisfy my needs. I have also used a 1Ds III and the live view is great when shooting with the X2. Any lens has to have manual aperture control, so the Mamiya D lenses would be almost impossible to implement. Wide angle options are expensive and limited.
I ultimately abandoned the X2 on a daily basis because it was too bulky and slow for my style of working. If my camera lived on a camera stand and all I did was table top, I think it could be a great tool. The X2 is extremely well made and is a great piece of engineering, the gearing is fantastic and it is a pleasure to use. There are some inexpensive part on the Calumet used sites as well.
Dave