Pros and cons - if you NEED the reach, you have to get a teleconvertor or have access to lots of cash for a long prime lens. I use a Canon 2x with a Canon 300 f4 and am very pleased with the results -
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Photo © Colin K. Work
The alternative is approx £8,000 for the Canon 600mm f4!
But Chris makes a very important point - many cameras will not autofocus with a teleconvertor attached - one of the reasons I chose the EOS 3 for my replacement system was the fact that it could still autofocus with an effective aperture of f8, but this facility is rare, and it must be said that autofocus is much slower with the convertor attached.
But that aside, image quality should not be a problem if you buy a top quality convertor. Experience with "bargain" convertors suggests these are a mistake.
Colin K. Work, Pixstel