The 1.4x teleconverter from Canon, when paired with a 100-500mm lens, effectively extends the focal length to 700mm. I have used the converter a few times but have not done much in the way of quality comparison. While shooting some Common Murres, I had the chance to capture some images from the same location with and without the converter and do some pixel peeping comparisons.
The additional detail in the extended image is impressive with no loss in sharpness (to these eyes). The biggest downside is that you have to extend the 100-500mm lens to a minimum of 300mm. That is fine on tripod but makes carrying it around for handheld shots more awkward. I definitely need to take more advantage of it for tripod shooting.