Only negative I saw with this lens: not sharp at f/1.2 within about 3 feet. After seeing this, subsequent testing determined it is definitely backfocusing. With a smaller aperture the mis-focusing is not obvious because of the increased DOF, but close shots, focused on the eye - were not sharp. Since that will be my primary use for this lens - low light, close in - this is no good for me.
A friend has the same lens and he's happy with it. We tried his lens on my 5D, and didn't see the problem.
Otherwise, great bokeh, wonderful performance in low-light situations. If not for the focus problem, I would love it - but I have returned the lens. I will either purchase elsewhere, or try the 85mm f/1.2.