I purchased my copy of this lens through Cameta Camera and used it for about 6 months before selling it recently on eBay. Previously I had been using a EF 50 f/1.4 with my 10D. Once I got the lens, I found myself using it more frequently than the 50.
Overall, I was very happy with the lens in terms of build, image quality and responsiveness. And the price is hard to beat.
My gripes (other than the ones mentioned in the summary) are relatively minor:
1.The lock switch. Sometimes I'd forget to lock it, and find the lens extended in the bag. At other times, I'd forget to unlock it. So I'd have to fumble a bit to unlock it, which sometimes made me miss a shot.
2. The lens cap. It has a tendency of coming off. Sigma needs to rework the design so it stays on snugly.