out of 5
I have the non-DG (extra coating) version of this teleconverter, and am amazed at the sharpness, contrast, and high quality I get.
I use it with my Sigma 70-200/2.8 EX HSM and the additional magnification is critical in certain situations. In 35mm terms, I'm getting a 400mm f/4 lens... and boy, is it sharp, even wide open. It's even better at one or two stops down.
If you have a GOOD long lens and you want to make it a little longer, for not much money, then this is a great choice.