I use the 1.4x TC on both my 70-200mm f/4L IS and 300mm f/4L IS lenses and it provides quite good results.
The 70-200mm is turned into a 280mm lens with an f/5.6 aperture. The 300mm is turned into a 420mm f/5.6 lens.
While the A/F and image quality is very good; the 300mm f/4L IS used alone beats the 70-200mm f/4L IS + 1.4x TC in both A/F and image quality.
The 400mm f/5.6L lens beats the 300mm f/4L IS + 1.4x in A/F and image quality.
However, the margins with which these two lenses wearing TC's beat the bare lenses is not very large. I am very happy with the results of the 1.4x TC on either of these lenses. if I were not happy with the results, I would have sold the 1.4x TC a long time ago.
I am so confident of the 300mm + TC performance that I left my 400mm lens at home and carried the 300mm + TC on an Alaska trip. Unfortunately, the weather was so bloody bad on that trip that I did not use the combination at all.