I have never been a fan of converters but I have used the 1.4X on occasion. This morning I went over to the Mississippi river to photograph eagles and with the distance to the eagles, which were staying on the inaccessible Iowa side, even an 800mm lens needs help from a teleconverter.
With the 1.4X added, framing is definitely more difficult for me and is more of a challenge for the excellent AF system of the 1DX III which was handicapped by my lack of tracking skill. The EF 800 bare is definitely an easier beast to manage for both me and the AF system but when I did my job the camera did what it should with the 1.4X and did it very well. The bonus is that my EF 400 f2.8 is going to seem so easy to manage for football and soccer season after this second (and probably for the year final attempt) to photograph eagles with a very long lens setup. It was tripod mounted on a Manfretto 161Mk2B with gimbal head.
This was my favorite shot of the morning @ f8, 1/1600, ISO 500 and cropped from about 19% of the 1DX III sensor area. Converted from RAW by Canon DPP which also applied lens correction, no additional processing. I felt sorry for the fish, it was like photographing a murder... And I still don't like using teleconverters even though sometimes they are a useful tool.