Having rented the 85 f/1.2L ii for a brief time in 2010, I will echo the above sentiments that state it is too specialized (the AF is too slow due to the full-time focus motor) and priced too highly, to be of much practical use.
However, if you have the need for slow paced photography at this focal length, and need a lot of bokeh and rich lovely color, by all means this is the one for you. I mainly used it on a crop camera, but also briefly on a 5D2. It vignetted more severely than every other lens I've tried, at f/1.2 or even f/1.4 (or any other f/stop), on my crop camera. So the vignetting on a full frame is even more severe. It wasn't as sharp at f/2 or wider aperture, as my 135 f/2L, at its widest of f/2. However by f/5.6, the 85 f/1.2, was as sharp as the sharpest supertelephoto I have tried...if not beyond it (and a bit sharper than the sharpest my 135 can go).
If my work needed the 85mm focal length often, I would probably first try the Sigma, (and would definitely buy the Canon f/1.8 regardless)...before purchasing the Canon f/1.2. As for now, I bought a manual-only Rokinon 85 f/1.4, and don't feel the need to buy anything else near this focal length. It's very sharp wide open, very little vignetting...color and "global contrast" a bit muted by "L" standards, but easily recovered in post. Also the "bokeh fringing" (mostly purple) is less pronounced than the Canon f/1.2 at its wider apertures.
If I ever have the need to use the forthcoming high megapixel full frame camera, then the 85 f/1.2L, when closed down quite a bit (perhaps at least to f/4.5), would be one of the few lenses that could make use of such a camera's resolution, even if it's 60 megapixels. Wide open though, you're fine with 20 or 22 MP. The color balance via my crop camera, was very vivid toward the violet and blue end...which yielded a very interesting palette. The only overall color palette from a lens I have liked better, was the 200 f/2L.
The 85 f/1.2L ii is referred to as "the magic canonball"...because it is almost round like a ball, and about as heavy. The balance felt very front heavy, and might even feel a bit too front heavy on a 1 series body. Certainly it would be the most balanced on a 1 series, compared to smaller bodies.
It was definitely a joy to use for doing landscape, and some shots of my family, and cat.