The last few years I've only used L series lenses, they give me everything I want image wise and moat importantly for me, they last the test of time (I always see them as an investment) and I didn't really see myself buying anything else.
Last weekend I was having a play with a canon 85mm f1.2LII in a store, like all L series glass I was really impressed with the build, but the AF and really soft manual focus ring really didn't do it for me. I do like the 85mm focal length, a few years back I had a canon 85mm f1.8 and often fancied an upgrade but the performance of the 85L and then the 85LII weren't quite what I was after. I shoot a lot of events/portraits in low light and need fast AF and a lot of reviews said that was a problem with the canon 85mm L lenses, despite the amazing IQ and sublime bokeh. I always thought I would like one, if only the AF worked a bit better. I one day hoped for a canon 85mm LIII.
After playing with the 85mm 1.2LII I noticed the Sigma 85mm 1.4 and picked it up to have a play and to be honest I wasn't expecting much, but after a few minutes I was seriously impressed. Build quality wasn't quite "L" quality, but felt pretty solid. The AF was great, the manual focus ring felt good and the IQ looked great on the back of the camera. I wasn't using my body or card, so no way of examining close up shots, but my first impression was that the Sigma 85mm 1.4 is a serious bit of glass.
I read a few reviews by photographers who used the Sigma 85mm lens for weddings/events/commercial shoots and were so impressed they ended up using it over their canon s85mm 1.2L or even 85mm1.2LII copies. They agreed that when they got the shot "right" the 85mm 1.2LII was better, but they had a much better hit rate with the Sigma 85mm.
Also, when you consider that for the price of a new canon 85mm 1.2LII you can get a new Sigma 85mm 1.4 and a new canon 135mm F2L, it does seem like a sweet option.
I can't say anything for sure as I just had a play in a store and read up a bit on them both, but would be nice to hear some experiences of others who have used both these lenses.