I've used this lens. I rented it from LensRentals. It is large and heavy. It takes a lot of patience. Stitching is fairly easy, but since the lens only moves in one direction you have to turn it 180 degrees to finish a stitched pano. I never could get it to tilt and shift (together) the direction I wanted it to go!! The dials are much too narrow and impossible to turn with gloves on in cold weather. IMO, you are much better off with a Nikon 85 PCE for this focal length.
Addition: I didn't see Allegretto's response before I rattled this off. He seems to feel the image quality is good. I re-checked some of the pano's I made and the color was great, better than a Nikon, but I don't feel the Schneider was any sharper and the copy I used also back-focused.
Yes, the 90 is very stiff! Both Nikon and Canon samples. Got the 50 today and it is much less stiff...
When you say "back focused", you mean the Schneider seemed to have more DOF behind your best focal point than in front?