I've got a Canon extender and only added advantage I'd add is that it reports accurate focal length information in the EXIF.
That's the older dg version, the newer dgx version reports correct exif. There's always a lot of confusion about the different Kenko genrations, just get the latest one :-p
Strangely, my 3 year old DG version gives me correct EXIF info. l just checked an image and it reports the lens as " EF70-200mm f/2.8L IS II USM +2.0x", focal length as 380mm and max aperture as 5.7.
Is this only because it's a version II lens?
Hmmm, I'm quite sure my 70-200 f4 IS reports the correct EXIF with either Kenko TC. Certainly my 300mm does. Only with the two stacked do things go off tangent - EXIF only reported for lens plus TC directly attached to it and ignores the one at the body end.