Don: One thing to know about teleconverters in general is that you do have some light loss -- usually about one f/stop with a 1.4x TC -- so that often it is better to use them in situations where there is really good, strong light on the scene. A TC offers advantages -- by putting one on a lens one increases the reach, which can sometimes make all the difference. They're small and easy to carry along in the camera bag, ready to slip on at a moment's notice, and just as easy to remove when not needed. One does have to check compatibility with lenses, as not all can work with them.
I use a TC on my 100-400mm quite often now, and that gets the reach out to 560mm. This works well for me when walking around my neighborhood capturing the wildlife (mainly birds and squirrels) or when I go to a local botanical gardens. If need be, I can take the TC off the 100-400 and stick it on the 200-600mm (which primarily lives on the tripod) to extend my reach with that lens, too, out to 840mm. Since both of those lenses are fairly slow, in terms of shooting in cloudy or not very bright days it's not as beneficial as using a TC on a prime 400mm or 600mm but it does the job on days when there is lots of sunshine and natural light.
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