I too only had one for a short time albeit it was on a crop. My copy had to be WAY stopped down to get sharp, hardly a good use for a 1.4. I'll try out a Sigma one of these days until then the flapjack and 24-70 do me just fine. (funny though how one is a feather and the other a brick.)
I took some tests the day I got mine, filling the house with the frame and focusing on the roofline. The white plumbing stacks towards the corners had white "shadows"/smears coming sideways off them through F2.8 it was so soft in the corners. The trees in the corners were garbage till hell F8 probably. So just now I took this sigma 1.4 out and did the same thing to see if it at least wasn't going to be worse(with my only real hope was to not have the coma stuff at night with stars/lights). Holy hell what a freaking difference. And it is cloudy and windy, yet just the tree(moving in wind no less) difference in the corners way open is so silly to what the Canon was.
I just went full frame again recently and now have 3 new lenses to sorta "review". 14mm Samyang, a 24L that was pretty damn far off(decentered and tilted focal plane) that I got used that is now fixed, and this sigma 50 I can compare to the canon 50 1.4. Been getting some pretty big eye openers through this. But now I want to see how close to typical the full frame corners were for that Canon 50 F1.4 to what I just briefly had. It was at least a sharp lens way stopped down. I was scared swapping it for the Sigma I might go backwards, but just looking at the zoomed in samples on my cam with the Sigma of the same scene......it's crazy crazy better.