I don't think the autofocus is as important as being able to control the aperture from the camera
, not to mention EXIF reporting. I have the Kenkos, and I really wish I'd gotten
the Pro Optic set instead, since I have an EF-S 60mm f/2.8 Macro, and I can't mount it on the Kenkos (the Kenkos only take EF lenses). Not to mention the Pro Optic set is about $85.
Here's a shot of a quarter taken with the EF 50mm f/1.8 II mounted on all three Kenko tubes:
I've put together
a Flickr set demonstrating the tubes, a diopter lens, and lens reversal.