Just obtained a used copy of a Sigma 100-300 F4 HSM lens, from MPB Photographic.
I'm impressed with the service from this company, which I haven't used before. Delivery was next day and the condition of the lens is, if anything better than described.
As for the lens itself, I haven't had the opportunity to do much more than 'play' with it and try a couple of test shots so far. But, initial impressions are very good.