There were a lot of complaints about quality control at Sigma a few years ago. I took me a long, long time to buy the Sigma 10-20, f4. I know of one very experienced pro that took his back three times before getting one that was OK! There were some complaints about Pentax at the same time. It all seems to have settled down now. It would, though, leave a bad taste in your mouth dropping that amount of money. I would also be a bit worried about buying Sigma lenses 2nd hand.