I bought this lens based on several reviews suggesting that I might be able to get a reasonably good copy that would be roughly as good as a good copy of Canon's 100-400 dust buster. Unfortunately, the copy I received just didn't cut it. Nothing was critically sharp and there were subtle halos surrounding the sharpest features.
It went back in trade for a new Canon 100-400, at a lot more money, but the Canon is beautifully sharp even at the longest focal lengths. The 100-400 is even sharper than by 300 f4L at 300mm!
Sorry, Sigma, I really tried hard to give you some business. I suspect there are some copies that are very good, however.