I am one that suffered with the 50mm 1.2L's focusing issues. It would be dialed in with MFA for closer focusing distances, but then it would be way off for longer focus distances closer to infinity. Also it would most of the time not lock focus on the subject. I would have to focus on something else and then re focus on my subject to get good results. Not sure if this is the "focus shift" folks talk about.
If my copy did not have the above mentioned issues, I would have kept it. It produced lovely results at f1.8 and f2. Mine was kinda soft at 1.2 and also suffered a bit of chromatic aberration.
No that I've moved to Full Frame and have a 35mm 1.4L and 85mm 1.2L II, I'm not sure that a 50mm is necessary.
Cams: Canon 1Dx, 6D, EOS M
Zooms: 17-40mm f/4L, 70-200mm f/4L, 24-70mm f/2.8L II, 70-200mm f/2.8L II; Primes: 22mm f/2, 40mm f/2.8, 50mm f/1.2L, 100mm f/2.8L, 135mm f/2L, 300mm f/2.8L IS II, 600mm f/4L II
Support: Gitzo GT4542LS/GT2830/G2258, RRS BH-55, Wimberley WH-200