Hello Colin,
In my experience, autofocus becomes unbearably slow and unreliable when you have a TC on. In many cases the AF will hunt endlessly, unable to focus at all. I've used one of the Kenko Pro TC's, as a Canon TC won't work unless you "tape the pins", which is something I was never interested in trying. As for the image quality, it's still pretty good, particularly when you're shooting with lots of light. But just as you've heard, there is a noticeable and disturbing degradation in IQ, particularly when you zoom in to look at your file at 100%. To my eye, degradation is not noticeable when I use the same TC with my 200mm prime lens. Speaking of light, everything you've heard is quite true; you'll need plenty of light to use this lens with a TC on. I've had some decent luck with slow-moving subjects when shooting on brightly-lit days, but it's certainly no fun using a lens that all of a sudden performs so poorly. A TC will truly cripple this lens, in my experience. I too wish there was a cheaper and less backbreaking way to get to 500mm and beyond, but one cannot have everything...
