I had the first version of the 1.4x and 2x when I still had the 70-200 f2.8 L IS. Image quality using the 1.4x was fine; using the 2x I noticed a dramatic reduction in sharpness. Ultimately I sold all three because I wasn't using them.
Over the years I've had a number of teleconverters (Canon FD and Minolta MD, as well as EOS) and rarely used them. I don't bother with them now.