Horrible is an Understatement!
Avoid this lens at all cost. After months of reading and watching reviews, I decided to go for this lens. I think the reviews that various websites and magazines give is by testing it out for a few days. I've had this lens for just over 3 years. It worked fine for a few months. Actually, it was my favorite lens (I have three other different lenses). The lens struggles to autofocus. I have sent the lens to Sigma 4 times now. Each time I pay for the shipping. Every time they say the same. They could not replicate the problem but still end up changing the gears. Their customer service is the worse. To give you an example, I had issues with my Canon lens, and they simply send me a replacement. Sigma refuses to send me a replacement even though their policy clearly states they will replace if they cannot solve. I even posted a video of the problem and they cannot fix. This is beyond frustrating. One of the worse customer service experiences ever! If you like headaches and being on the verge of calling you a liar, then buy this lens, otherwise look elsewhere.
Pros: Great when it works.
Cons: Auto focus issues and dealing with their horrendous customer service.