Love my 100-400mm!

Rocco, the comparison you linked isn't relevant for two reasons - one, it's the non-OS version of the Sigma, the OP is asking about the OS version. Second, you've got two different cameras selected in the drop down - if you pick the 1Ds II for both (that's the only one on which Bryan tested the non-OS Bigma), the IQ difference isn't that much.