Garry,
I shoot Nikon, not Canon, but I am sure they are very similar. Once you get past f/5.6 or f/8, you will lose AF. Most of the teleconverters do not work well on zooms, but there are some notable exceptions. With the Nikon's, the 70-200mmf/2.8 and the 200-400mmf/4 work marvelously with teleconverters.

I would defintly think of the TC's as second to a good lens- they are supplemetary- lens first tc second- one thing this has taught me is that range is second to clarity, if that makes sense...........the new Canon 70-300 IS would be better than a tc IMHO- then a TC.
Also, to add to my earlier lessons learned,the cheaper Tamron is auto focusing and working with almost all my zooms- cheaper was better here.... but the lens needs light, and TC's are thiefs.........