I'm biased, Sigma USA has treated me like I am a million-dollar customer. I use a Sigma SD15 DSLR along with a 7D. My first DSLR was a Sigma SD9, then and SD14 and now the SD15. My first and only Canon is the 7D (unless I was going to go full frame, there is no reason to switch from a 7D). I have 11 Sigma lenses:
50mm EX Macro (Sigma SA mount)
8-16mm DC HSM (Canon mount)
17-70mm DC OS HSM (in Sigma SA and Canon mount)
105mm EX DG Macro (Sigma SA mount)
70-200mm EX DG (Canon mount)
80-400mm EX DG OS (Sigma SA and Canon mount)
18-200mm DC OS (Sigma SA mount)
18-250mm DC OS HSM (Canon mount)
24-70mm EX DG (Sigma SA mount)
I had issues with two lenses, the 24-70mm and the 80-400mm. My wife accidently dropped my camera with the 24-70mm lens on it and damaged it. There was a nasty grinding noise when zooming the lens. Sigma repaired it free-of-charge even though I made it clear how it got damaged. The 80-400mm SA mount lens stopped focusing correctly. The lens has been out-of-production for some time so there was a good chance that no parts were available. Fortunately, there was a box pf parts for one lens on the shelf. The tech installed every new part they had into m lens. The repair was at no charge, even though the lens was long out-of-warranty.
I'm fortunate to live nearby Sigma USA so I can visit them when needed. My first DSLR, the SD9, was experiencing some AF problems. I was about to go to see my family in Greece and was concerned. They offered me a loaner while they had my lens but in the weeks before the trip, the camera seemed to be working so I kept it. The AF went wonky on me towards the end of my trip. I brought it to Sigma and they had to send it to Japan for repair. They offered a loaner and in six weeks my camera was back good as new. It was out of warranty by three years but there was no charge.
To top this all off, the Sigma EF-500 Super flash I had for my Sigma SD9 and SD14 was not compatible with m SD15. Some units could get a firmware upgrade but others could not and I had one of those. It was the president of Sigma USA who gave me the news. He offered me a new EF 610 Super flash at a discount. I was very appreciative but told him that I had to turn the offer down as I was out of work and couldn't justify spending any money on camera gear. He asked me to wait and then got the flash for me and asked me to take it as a gift. How cool is that? I also had the good fortune to meet the late chairman, Michihiro Yamaki, some years ago in Vermont. If any person could be described as a "true gentleman", it was Mr. Yamaki.