Based on the reports so far, and my experience with the 35mm 1.4 Art, I'm looking forward to giving this lens a good working over when it comes out.
I have had my Canon 50 1.4 since the mid 90's, and loved it in the film and early digital days. But over the years, as my L lens inventory grew, I used it less and less. It's sharp enough in the mid apertures, but wide open never has really satisfied me, especially in the last few years when the 5D II and 5D III showed its weaknesses. When I got my 35 1.4L around the turn of the century, the 50 1.4 has pretty much sat in my bag, with a 28-70 and then 24-70 L zoom handling the focal range at mid apertures.
I like the 35 1.4 Art so well that I'm seriously considering giving up my 35 1.4 L. I'm keeping the Canon for another year or so while I see what Canon does for an encore, and while I see what the long-term reliability is for the Sigma. Right now, I'm really pleased with every other aspect of the lens, including all AF-related areas of performance.
Again, if this 50mm 1.4 from Sigma is anything like the 35, I'll be a happy camper. For sure the Canon 50 1.4 will be gone in an instant. It is a necessary evil at this point, even though I love a large aperture 50mm from an artistic standpoint.