I had a mark 1 that lost infinity focus and couldn't be repaired unless I could source a donor lens with a different fault.
By that point I threw the towel in, optically there is no difference in any case.
I wouldn't buy a mark one now, mainly because of the parts issue, 23 years is a lot of iris movements etc...
They have become a cult item, fair enough, i just kind of think if you are going to spend 50% more for build quality then why not go that little bit further and get improved build, improved focus, improved bokeh, improved max aperture and pick up an in warranty in production servicable 1.4 instead?
I settled for the plastic fantastic.
Actually from what I've heard the 1.4 is not really that reliable, with stuck focus rings for instance. History shows the Mk I to be remarkably durable, so it's worth having a unit that hasn't seen that much use over the years (hooray to my sublime copy
Nothing wrong with the plastic fantastic by the way, given its low pricing (but still I prefer the Mk I).
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses