Great lens, spotty Sigma QC
Great telephoto, was lucky pick up a new copy rather cheap. Feels great, finally Sigma decided to drop the old finish. Optically its better then Mark I Canon, but worse then Mark II Canon. If you can Afford Canon 70-200 IS USM II go for that instead. OS is very reliable (so far) had to send it to Sigma once for AF repair because it stopped working (after 3 months of using it). That is one thing that you always have to keep in mind with third party lenses. Out of all the lenses I had, Sigma are the only ones that I had to send for service.
AF stopped working, had to send it to Sigma for repair. They fixed it, question is for long.