I was happy with Kenko extension tubes, until i used them to hang a 1D off the back of my 500mm. There was a bit of flex, and it made me nervous. Now for the big jobs I only use the Canon tubes. The "air" is the same, but the tube is a good deal more solid.
I have not felt the same way about the Kenko t-Con however, but having seen it in the tubes, I can understand the concern.
Ummm. I've never noticed any flex with mine or paid attention to it maybe.