I tried 3 copies of the Sigma 35A until I got a good one. But boy, is it good.
When it came time for me to buy a 50mm prime, I thought I wouldn't put myself through the Sigma AF drama. So I bought a 50L. After a couple of weeks of using it, I sold it. I was really disappointed in how soft it is wide open, especially compared to the 35A.
So I took a leap of faith and bought a Sigma 50A. AF is perfect right out of the box, and the image files are beautiful. Painfully sharp wide open. I don't feel I'm missing anything from the 50L. I don't see any difference in "magical rendering" that people talk about.
Looks like Canon zooms (I have the 24-70 II and 70-200 IS) and Sigma primes for me. Can't wait for the 85A!