A big part of those accusing Sigma dont even own one. They just repeat the same old threads. While I do agree that older Sigma lenses had noisy AF and QC problems, they were also a third of the price of Canon/Nikon. However now that I own a new (and expensive) Sigma lens, I can say that this baby is something else. Silent, fast and reliable. It's now my best lens and I can only say that if people skip it because of Sigma bad rep, it's their lost.
For any lens you buy, just be sure you have a return policy in case something is wrong with the lens. For fast primes, having MA on your body is a must to compensate the gap in body/lens calibration.
How big of a part? Care to back up your claim with data? Or is this just repeating something you read off the internet. BTW, I don't currently own any Siggy glass so maybe I fall into that category. I tried to, several times but got burned so bad they they have made my life long ban list along with Apple and Sony.