Just bought the Sigma 50mm 1.4 today and did a rash of test shots. Much of the inspiration for purchase was the bokeh improvement I saw examples of, above that of the Canon 50mm 1.4 which I sold. The Canon 50mm 1.4 did not disappoint me .. it's just that I knew I could do better for not too much money.
Will probably move to the Canon 50mm 1.2 eventually. But that one won't match my Sigma 85mm 1.4 which I use most for portraits, and which proved side-by-side recently to be as good as the Canon 85mm 1.2 that I tried. Since the 85mm will be my main lens on FF, the 50mm is intended as a backup on my crop sensor ... so it seemed okay to go for the $499 good lens with a $100 instant rebate.
My first impression is that the bokeh of the Sigma 50mm is genuinely better than the Canon 50mm 1.4. The Sigma likewise is a bit soft at f/1.4 like the Canon, but a teeny bit better at f/1.6.
My copy seems to focus spot-on. But about 1 out of 12 times it didn't get it right at all. Which makes it pretty good, and twice as good as the Canon 85mm 1.8 that I used to have.
So for $399 ... it's doing what I hoped for regarding focus. And then .. an improved bokeh was 80% of my reason for getting it. So all in all I'm happy with the purchase.
Although use of filters would be minimal, it seems rather nice that this Sigma 50mm, plus my Sigma 85mm, can share the same 77mm filters as my Canon 24-105mm