My thoughts regarding teleconverters in general based solely on personal observations and some edumicated guestimation. Comments below stated declaratively done so for simplicity, not necessarily on empirical knowledge
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TC's are akin to adding a filter to the lens.
Any time you place additional glass in front or behind the lens you are going to have a decrease in IQ. The absolute best image you can get is with the bare lens. Adding a teleconverter to the back of the lens will amplify any imperfections, softness, etc. inherent in the lens itself proportionately with the TC's multiplier.
Teleconverters steal light, robbing you of 1 or more aperture stops. Focusing becomes more difficult, and image stabilization becomes an issue.
And like filters, the quality of the optics can play a big role in how cleanly the 'finished' image strikes the sensor/film.
I have the Kenko 1.5x TC and I use it very sparingly now. For one, I *rarely* need to reach out past 300mm for a shot. But most importantly I realized after the "wheee factor" wore off a bit that I'm not printing my shots as posters and in most cases I can take a 200mm to 300mm shot, crop it and still get a very good image and/or print.
Again, I'm no expert and at most as far as photographic skill I would call myself a "cerebral novice" - though I have aspirations for greatness.