The Canon is good for small prints and focuses like the wind--in the highest class of fast focus. But it's not useful as a picture-making machine below f/2.8 due to the low sharpness, low contrast, not to mention the purple fringing (I don't really care about that, since it is for the out-of-focus areas--I just care that the in focus areas are extremely sharp, and they are not sharp below f/2.8 ).
There are a lot of inaccurate reviews of the 85mm f/1.8 saying it is sharp, but they're using it in a variety of circumstances shooting on automatic and most of their review photos are f/2.8 or above, lots of f/5.6 shots, etc. For those the 85mm lens is world class. For this reason I own two copies, and two copies of the similar 100mm f/2.0 as well. The same comments apply to the 100mm f/2.0 lens, despite the fact that Ken Rockwell says the 100mm f/2.0 has 5.0/5.0 perfect optics. He is talking about photos that are not near f/2.0.
But when I need those apertures below f/2.8, then the Canon f/1.8 is simply not an option. Occasionally a picture is good enough at f/2.0 that I think there is hope, but it just isn't possible to call it sharp. The fast focusing sometimes makes up for it. To be fair, I made a 20x30" enlargement for someone two weeks ago that was shot at f/2.0 with the Canon f/1.8 lens. It can be done, but it's not pretty.
The Sigma on the other hand is three steps backwards in terms of focusing speed, but that still puts it far ahead of the 85mm f/1.2L in that area. The one problem that I hate about my copy of the Sigma (I've had it for 1.5 years and shot hundreds of events with it) is that it just doesn't do focus tracking properly. It focuses plenty fast enough even to take basketball photos, but it doesn't track movement after focusing, but waits for a moment. So I have to keep my finger off the shutter, and then push it at just the right instant so that when it is locking on it will actually track the motion in order to lock on, and then I get a perfectly focused shot.
It has no trouble locking on to a moving object, but it would be a lot easier if it would keep on tracking it so I didn't keep having to lift my finger up and down. I think there is something messed up with the algorithm in my first lens. I have another one arriving this week (20% off from Amazon with the purchase of a Canon body that I ordered) and I am hoping that it will be different.
Image quality from the Sigma f/1.4 is unbelievably good. That's all there is to say. Some tests have shown that it is better at f/1.4 than the Canon 50mm f/1.4 lens is at f/5.6.
In theory that's possible since the effect of diffraction is much more negligible at f/1.4, but in lens manufacturing it's pretty hard to make a diffraction limited lens faster than f/5.6. (Diffraction limited means that the lens design is so good that the only factor limiting resolution is diffraction. If this were the case with an f/1.4 lens, then it would have resolution 16 times higher than an f/5.6 lens.)