Gonna play a little Devil's advocate....
I was in photo retail, in a pro oriented camera shop in the 1970's up to the mid 1980's. At one time, our little 15x55 foot shop did 1/2 of one percent of all of CUSA's North American photo business, or, put another way we were in the top 200 of ALL Canon dealers, including mass merchandisers. Canon themselves described our volume as "tonnage".
At that time, "the golden age", there were a lot of garbage "off brand" lenses, but also a few good ones. One that I particularly recall was the Vivitar Series 1 70-210, and the Series 1 90mm macro. Those were made by Kiron and/or Tokina and/or Cosina, maybe all three... dunno.
The point is (I'm getting there), these were actually very good lenses as compared to the "then current" competition, even from the likes of Pentax, Nikon and Canon. They got a lot of praise from local press photographers, and photo educators (which all knew, and did lots of business with our shop).
My beef then, and the beef I still hold today is one of contrast and color that is consistent with the lenses from, say, Nikon or Canon. While the very good accessory lenses were sharp, and even exceedingly so, or faster, or both... they just didn't have the same snap and color. The two I just cited were rather blue and "flat" compared to the very warm Nikkors of the day, and even the rather neutral Canon FD lenses. This was something the late Steve Kippert (my Leica rep) showed me, and its something that has stuck with me all these years.
I know these days, color and contrast are a snap to alter, but I'm still wondering ... based on the same settings, no alteration of the images, how does the Sigma 35/1.4 compare to the entire (and rather consistent) Canon line?