just got a Kenko 1.4X Pro, primarily intended for use with my 100 macro
in order to do a side-by-side comparison with my Canon series 1 1.4X i used them both on the 90TS which performs about the same as the 100 macro and allows the Canono extenders - with the 5D i could see no difference between the extenders
also tried both on the 100-400 and 300 f4 with 20D - again no difference in image or autofocus
the Kenko performs exactly like the Canon (including requiring 3 pins taped on the 100-400 for autofocus)
out of curiosity i tried the Kenko on my 70-300DO -- it autofocusses! - but of course the images, which are nothing to shout about at long focal lengths anyhow, were as bad as anticipated
given the compact size, light weight, and slightly lower price, i'd recommend the Kenko over the Canon and will probably sell off my Canon when an opportunity arises