i've used kenko 1.4x tc b4 i switched to canon's... cheap and good at $220 +/- from ms color i think...
i regret selling it for the canon's coz to my eyes, opticlally no difference between kenko and canon's 1.4x... advantage of canon 1.4x is only the weatherseal(which i dun need) and build(heavier).... in addition, it can't af at f8 with a slow lens on a non 1 series body... need to tape pins... on a kenko, iirc, u can af at f8... it's lighter and cheaper....
cheers...