While Overread is correct, his explanation is a bit too far reaching in the way of image degradation. Perhaps a little more information on TC's would help.
TC's are designed to be used first on lenses with a focal length of 100mm or more. TC's are also designed to be used on prime lenses of high quality. They can be used on zoom lenses but they again need to be of high quality with focal lengths over 100mm. If you looked at the links I posted you will see that Canon's TC's only fit selected L lenses both primes and zoom. You will also notice that there are no primes under 100mm and the only zoom under 100mm is the 70-200.
TC's magnify the image of what ever lens it is on. Canon TC's will magnify either by 1.4X or 2X. When the TC magnifies the image that the lens sees it also magnifies the in problems that the lens has. If the lens is slightly soft, it will magnify that softness. So yes, it will degrade the image produced.
However, when a top quality TC is attached to a top quality lens you can get very usable image. I own both the Canon 1.4 and Canon 2X TC's and have no issues with using them. The only two lenses I use them on are my 300mm f2.8L and 400mm f2.8L. They are razor sharp lenses. The degradation the TC's may cause is very slight. Both the 1.4 and the 2X produce very usable images on both my 300mm and 400mm.
If you are interested in a TC I would offer two suggestions. First take you camera and the lenses you are interested in using it on to a good photography shop and take a few pictures with and without the TC you are interested in. Go home and take a look at the images they produce and decide if you are satisfied with the results.
Second, if you want that extra length, I would pass on the 55-200mm consumer grade lens and save up for one of the 70-200mm L lenses. The 1.4 works quite nicely on my 70-200 f2.8L. The 2X image quality on that lens is a bit less than I would want to use. Granted I rarely use this combination unless I happen to be in a situation where I don't have my 300 or 400 with my. My 70-200 goes about everywhere and my TC's are always in the bag as well.
Kenko's top of the line TC's are considered by many to be on par with the Canon TC's and will fit almost any Canon lens made. However, the same thing applies to the Kenko's as does the Canon's when it comes to light loss and image degradation. A top quality TC on a fair quality lens is still only going to produce fair to poor image results.
.
