Kenko Teleplus MC4 2X
Just started using this TC with my 200 mm F2.8L. This royal tern was taken today, ISO 400, 1/1600s handheld. Data in EXIF ignores the TC, reporting 200mm and F6.3, which is what I set to use in aperture priority mode.
Having no experience with the Canon or other TC's on a digital body, this looks pretty good to me. Build seems good, nice fit to lens and camera body with no play.