I love mine, took me 5yr to get it and not without pain/struggle. It's very weird, inconsistent and heavy but I think it's one of the few lenses I own where non-camera people look at the image and say, "wow, that looks gorgeous" pretty much without effort. It is not remarkably sharp, it has a LOT of vignette, and I think its sweet spot is around 4-5.6 in terms of overall beauty. But I would never let mine go.
Now be careful if buying one. Mine arrived with the zoom group rattling around inside. It was BAD, to the point where I couldn't risk shipping it back because that would ensure it was destroyed totally, and I couldn't send it for repair for the same reason. So I drove it to ZGC optics and Guy explained that because it's a modification of an old lens, there are zero parts available. You could try to find a 20-100 and customize some of the bits but more likely you're looking at repairs needing custom machining. AND, it's one of the most complex lenses Cooke ever made, so adjusting anything takes HOURS to tear-down, adjust, rebuild, project, ---rinse/repeat. AND even if you get it all aligned perfectly, it was hand-made and customized so it may NEVER have been a good copy in the first place.
I was stubborn and hopeful so I shelled-out and had him fix/calibrate and rebuild anyways, and he did a remarkable job and I lucked-out that nothing was actually broken, just loose. These are difficult to maintain so most owners never do - and things like the zoom group retainers can get loose due to the weight of all that glass rattling around in cases. Moral of the story - it's special in an odd way. It's not modern in any way, it cuts well with original non-recoated panchros. I personally like the look better than original panchros though not as much as the new panchros if that makes any sense?