Canon EF Extender III 1.4X on Canon 135mm f/2L
I've posted my first comparison using the new Canon EF Extender III 1.4X, showing how it performs with the 135mm f/2L against the Canon 70-200mm f/2.8L IS II and 70-200mm f/4L IS.
See my review in DAP.
The extender acquits itself fairly well, but you'll have to judge for yourself as to whether it fits your needs at various apertures.