Dustin Abbott has completed a very in-depth review of the Sigma 50mm f/1.4 ART series lens for Canon. There’s a written review, a video review as well as a sample gallery for you to check out. I agree with most of what Dustin says about the lens, especially the part where this lens is leaps and bounds better than the 50mm lens it replaces. I tend to use the Sigma 50mm f/1.4 ART more often than the Canon 50mm f/1.2L, as I find the AF inconsistencies from the Sigma are far easier to deal with than the focus shifting of the Canon 50mm f/1.2L.
“All in all, Sigma has made a major breakthrough in creating a large aperture prime with world class optics that is in a price range that most professional and many amateur photographers can afford. While the stated “target” is the Zeiss Otus, there will actually be fairly few photographers that are cross-shopping these two lenses. The Otus is still the optical king and enjoys a certain cache that Sigma can’t touch. But the Sigma is a far more practical lens for most photographers, as most photographers need autofocus for their work. The inconsistency in the AF performance is a concern, but the truth of the matter is that there are a variety of focus concerns with other 50mm lenses, too. The AF in the 50mm ART is snappy and (most of the time) accurate. I have already used the lens in professional settings and have been mostly pleased with the results. I wouldn’t hesitate to use it professionally in the future. This is an excellent lens that sets a new benchmark for autofocusing 50mm lenses.”