Excessive purple and green fringing in the range checked: f1.2 to f2.0. As of today, still have not checked the CA behaviour when stopped down further. The CA is brought about by any higher contrast boundaries and highlights, such as in the image of a crystal vase in a rather dimly lit room. The design of my 85 f/1.8 optics clearly controls the CA better.
The focusing lens group drive makes somewhat unsettling rocking noise in its target position, as if there was a degree of backlash or calibration hysterisis in it.
The manual focus ring feels rickety, not consistent with the solidly built remainder of the lens.
The location of the red "dot" for lens-camera mount alignment is inexplicable: one can not actually see the "dot" when trying to engage the lens onto the body.
In summary, what I see in this lens design is the focus group drive assembly, CA control and packaging having been made subordinate to lens speed. I would be hard pressed to see in this lens the sort of excellence I have found in some other (cheaper) Canon L and non L lenses. Having f/1.2 nominal aperture, this brute of a lens is just (obviously) fast and probably of a sharpness roughly comparable to EF 50 f/1.4.
So, do get it if you must or really want to have the f/1.2 lens speed (at 85 mm). If you can live with f/1.4 or even f/1.8, look elsewhere.