I have been researching the Canon 50mm f/1.2 and I eventually bought it and it's a lovely lens.
However it was harshly criticised in the fredmiranda forums from 2006 roughly when it was introduced up to mid 2009 when opinion began to turn around.
It seems that later versions of this lens have less issues but Canon have not made any public statement about it. I made sure that my lens was made in 2010 by checking the date code.
The main criticism seems that it auto-focuses poorly in low light and it's soft at wide apertures.
I wondered if those people didn't consider the very narrow DoF and the trouble that some AF systems have with low light. I would suggest that this lens be focussed manually at low light levels.
The other topic which is not fully understood is focus shift. This can occur even if focussing manually. You focus wide open, no problem, but focus at say f/1.4 or f/1.8 and you get things out of focus. The phenomenon is that you set the aperture at f/1.4 and focus with a wide open aperture and the lens stops down to f/1.4 and the focus shifts to a short distance behind the subject. At smaller apertures, say f/2.0 the DoF increases and focus shift is no longer a problem.
You can test this by putting your camera on a tripod, focus on a suitable object like a ruler placed at 45 degrees and then take photos but just change the aperture. You may see the focus shift slightly behind the main area you focussed on.
This is a problem with fast lenses and even the expensive Noctilux can suffer this. It is not a defect but a law of physics and is can be designed around. Apparently a floating lens in the design mitigates this. I would say that every lens has focus shift in one form or another but it's probably not a problem with telephotos, wide angles etc. Seems to just be more noticeable with 50mm fast lenses.
Around 2009 Canon may have modified the design to reduce the complaining. Might have been a factor in the decision to no longer produce the 50mm f/1.0.
I hope I'm not stating the obvious but a search of the forum didn't really mention focus shift with fast lenses. In researching the 50mm f/1.2 before buying it, I found it fascinating that it had such poor reviews probably arising from a poor understanding of the focus shift issue.
You can Google this topic in respect to other fast lenses such as the Notilux and the Nikon 50mm f/1.2 which was first made in 1989 and is a manual focus lens. People are also discussing focus shift with these lenses too. Indeed, name any fast lens and someone comments on the focus shift issue.
It's interesting that very few fast lenses are still in production, mainly because they are expensive. The other one which comes to mind is the Noktor which is the four thirds f/0.95 lens.
I think focus shift is a worse problem with large formats, view cameras and bellows type rigs (correct me if I'm wrong)
Anyway (as I said before) I found the topic fascinating and the Canon lens may have been unjustifiably maligned.