I have both the Canon 1.4xIII and 2xIII. I have read that the Kenko 1.4x is 'as good' as the Canon 1.4xIII in some reviews, others indicate that it's as good as the 1.4xII and a little short of the 1.4xIII. I have not read many reviews of the Kenko 2x, but the few I've seen suggest it's not nearly as good as the Kenko 1.4x or any Canon 2x.
The other issue with Kenko TCs is compatiblity. They come in different firmware versions, indicated by colored dots on the TC. They cannot be updated. Many users have reported issues with certain combinations of Kenko TCs and Canon bodies + lenses. One apparently reproducible issue occurs with the latest Kenko 1.4x and the 5DIII - if you have AFMA enabled, the camera locks up and you need to pull the battery to reset it. The 'solution' is to not use AFMA with the Kenko TC...and IMO, that's no solution at all unless you're lucky enough that your combo doesn't need it (and no guarantee that will be true with the next lens you try).
The only reason to go with a Kenko TC, IMO, is if you require it for a lens like the 70-300L or 100L Macro, where the Canon TC's won't work for physical reasons. But personally, I'd recommend going with the Canon TCs and sticking with compatible lenses.