Ben Pike , Jun 24, 2011; 05:31 a.m.
I have the Sigma on a 7D. It is big and bulky as you would expect and has to fool the Canon AF into working at f6.3, however I don't have much trouble with the focusing, but I think it would struggle with a converter if the autofocus works at all. The lens can give good results used correctly and is certainly worth the money, however that said if I could have afforded the 100-400 at the time, I would have bought it instead for build and image quality. To be honest with you, for my needs I am likely to sell the Sigma soon and get the 70-300L. With a tripod and stopped down, the Sigma is capable of good results, but I suspect you'll always have doubts over whether you should have got the Canon.
One further note on the practical side, I have found it impossible to use video with long lenses. Not that I need to, but try holding the Sigma at arms length and track a subject using live view and you will see what I mean.