I use the extenders on the new 70-200/2.8IS II and the old 300/2.8IS and 500/4IS, mostly
with a 5DIII or 7D.
As already stated by some others:
Optically the 1.4x was not improved significantly but the Mark III 2x extender was really a change!
Before I used the 2x only in cases of emergency now I use it quite often on the zoom when I dont
want to carry the long stuff or on the 500mm for little birds (with workig AF on the 5DIII now!)