I tried a Kenko 2x for a short while but wasn't too impressed with it. I have the 1.4x Kenko and I also have the Canon 1.4x MkIII and the 2x MkIII and without a doubt the Canon will give you better results on a 100-400. As said above, you will lose AF with the 2x but for moon shots and other long distance (specially static) where you can manually focus it's brilliant. I've not tried the MkI or MkII Canon extenders so if you can try one of those and get one you're happy with it you might get it cheaper.