I used to have a set of Kenko tubes and would disagree with this, while they worked fine with a number of my lenses (105 macro and 400 f5.6) they did give problems when used with IS lenses (100-400, 300 f4 and 500 f4). While the allowed the AF to work perfectly the IS went mad, it juddered alot. Needless to say when using with IS lenses I turned the IS off, but this was enough for me to sell them I'll be buying Canon tubes when funds allow. I had the same problem with both Kenko and Sigma 1.4x tcs on my 500 f4 but with the Canon tc it's never been an issue, I don't know what causes it but for me it's reason enough to stick with Canon gear.