I am so, so happy with that extender... it works great with my Canon EF lenses (100/2, 200/2.8L, 300/4L, 400/5.6L) adapted with a Fringer adapter.
The only 'minus' is that the EXIF file records the original focal and aperture, not the focal and aperture of the combination extender + lens.
Image quality is fantastic, AF is working fine (well, a little bit hesitant).