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It's a hard one.
You lose sharpness and add some CA with these extenders - so at some point, cropping and interpolating images without the extenders make up some of the ground you get by adding an extender.
I have a 1.4x for the 135mm f/2 for indoor sports and stuff, but I really never use it.
Still, some of the top nature / wildlife guys use them all the time and swear by the things, so I'm sure it's worth it if you're shooting the right subject and have the right gear.