Just a comment on why TCs sometimes kill autofocus. The better TCs have some way to tell the camera that they're reducing the maximum aperture of the lens. This is specifically for Canon SLRs, I have no idea what happens with Nikon gear, but beyond I think F5.6 or something the camera won't try to focus. It figures there's not enough light and doesn't even try. The infamous "bigma" (Sigma 50-500) cheats at the long end because the real maximum aperture is actually smaller than the limit for Canon cameras. So it reports that it's wider than it actually is, so that Canon cameras will still focus.
Some people do the same thing with teleconverters. I don't have one, but I've read about this. Apparently you can cover up two electrical contacts on the TC with tape, and then the camera doesn't even know the TC is there. So it won't know that the focal length is changed, or that the aperture is smaller, and will try to focus as it always does. It may not be able to if there's not enough light, but I think this often works. Also apparently the camera might slow down the autofocus process if the aperture is smaller, but not TOO small. The taping the contacts trick forces the camera to focus normally, which may work faster or may result in the lens having to hunt around to find the right focus point.
And just to repeat what I've heard about the TCs.. I think the Canon TC (I'm speaking of the 1.4, the word is that 2x is too much and causes too much quality loss, in some cases worse than just interpolating the final image to blow it up) is supposed to be good, but it only works with L lenses. I mean it only fits on them, you just can't plug a normal lens on to it. And while I don't remember the specific model, I think that a Kenko TC was often said to be a good TC that costs a lot less and still looks nearly as good. I may be confusing that with Kenko extension tubes though, which are a totally different thing.
This is based off of extensive reading of posts on the dpreview.com forums. I'm only going off of what others have said. I should probably get one, I'm in need of more reach and my 70-200F4L is supposed to respond to a 1.4 extension quite well. But I've just never done it.
