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Depends on what kind of glass your using them with really. For a lens that has an aperture of F/4.5-5.6 I wouldn't recommend them. If you have faster glass, like a 2.8, they work pretty well. I use a 2x with my 70-200 and it becomes a 400 mm at F/5.6, autofocus is nice and fast and it works great in decent lighting conditions.
With an F4 you can probably get away with a 1.4 x, they only drop you by one Fstop - but for a 2x you really need good fast glass.
I have a 2.8 prime I'll be using it with so it should perform well.