I've looked at the Canon 85/1.2 and Sigma 85/1.4. I'm mainly interested in shallow DoF photography of people.
The Sigma 85/1.4 seems nice, but concerning shallow DoF it seems like the Sigma does not at all go out of focus as much as the Canon, even at the same aperture.
Look at the girl at the fence in this review:http://www.thedigitalpicture.com/Reviews/Sigma-85mm-f-1.4-EX-DG-HSM-Lens-Review.aspx
and compare f/1.4 - f/1.8, look at the fence behind the girl.
For Sigma f/1.4 to f/1.8 the "out of focus look" is almost the same, just with different vignetting. For Canon f/1.2 and f/1.4 are rather similar, but there are much clearer differences in out of focus look f/1.2 - f/1.8. f/1.4 on Sigma looks similar to f/2.0 on the Canon, that is the Canon can give much more of the desired buttery background. Sigma's bokeh is nice, it is not just as smeared as the Canon.
Since getting shallow DoF -- kind of a medium format look -- is the main reason for me to get a 85/1.2 or 1.4 instead of the low cost Canon 85/1.8, I've become a bit skeptical about Sigma. It is indeed much cheaper than Canon 85/1.2 but if it still is a half-measure concerning shallow DoF, why not just go for the cheap 85/1.8 instead?
I've read that the auto focus of the Canon 85mm/1.2 is not really fast. Anyone who knows how slow it is? For people I'm sure it's ok, you don't need fast focus there, but is it possible to shoot indoor sports for example? Today I use a 70-200/2.8 II for that purpose.