This is a superb extender which works flawlessly with the Canon prime telephoto and super telephoto lenses. I bought it to supplement my Canon 300 mm f/2.8 IS II lens to give me an effective 600 mm f/5.6. The combination is light enough to hand hold and WAY cheaper than the 600 mm f/4 prime alternative offered by Canon. I am extremely pleased with the extender; I honestly can't see any degradation in the image quality when used with a Canon 5 d mk III body. Everything is corner to corner crisp and sharp and I see absolutely no sign of any contrast reduction in the images. It will also work very well with the popular 70-200 f/2.8 IS II although there is a very slight reduction in sharpness at the 70 mm end of this lens combination; the 200 mm end image quality is extremely good. If you own any of the Canon "big white" prime telephoto lenses, this is definitely a worthwhile accessory.
Pros: Very cost effective way of increasing your telephoto reach. It's very light and compact.
Cons: Loss of two stops of light compared with the mated lens.