I purchased this lens from Amazon.com and ended up returning it due to severe back and front focus issues. If I used my D700's AF Tune feature to compensate for the back focus at 500mm the 150-250mm range would badly front focus, with the reverse holding true, too.
This was sad since the lens was very sharp, even wide open, when focused correctly.
In the end I don't know if I was the victim of sloppy Sigma QA, or done in by the poor level of padding Amazon.com's shipping department used in packing the lens. The shipping box had airbags padding only two sides of the lens box. Since Sigma boxes the lens only in its nylon carrying case I'm sure the lens got its share of bounces during the four days it spent in the UPS system.
So if you're willing to do the necessary amount of exchanges to get a good copy, or are willing to send it back to Sigma to get tuned, it can be a great bargain for the focal range it offers.