The other day I was looking at Canon compatible teleconverters; I have a 70-200 f4L IS and sometimes would like some extra range. The ones that I am considering are Canon's own 1,4x (as I would lose AF with 2x) and Sigma's 1.4x. The Sigma is cheaper, but most people say stick to main brands, is the quality difference that big? And is there any other brand that builds a compatible teleconverter?
I have been looking for reviews on the web, but nothing too informative. Obviously you cannot really test a converter alone, you always test the converter/lens combo. It would be great to hear your experiences, especially from somebody who has used both brands (or others).