Fred, the answer also depends on your intended use. Any extender will somewhat reduce autofocus performance, it isn't just the extender adding to the overhead of the AF algorithm but also the fact that you are losing one or two stops of light which reduces AF speed due to reduced performance of the AF sensor in lower light. So it is more of a problem for action shots like sports or wildlife. AF on the better bodies will still work quite well but not as well as they do without the extender so it is one of the compromises you have to consider.
I have a couple of flavors of the Canon 1.4X (versions II and III) and both work pretty well with my EF 70-200 f2.8 and fast telephoto primes. I usually keep the 1.4X in a bag with me in case some unexpected need comes up but the only times I have used it in the last few years was for a few baseball games combined with the 400 f2.8 when shooting batters from behind the outfield fence and with my 800 f5.6 when shooting eagles and that combination requires a lot of collaboration between the camera and user to have the subject in focus at the exact point and time needed.
I also have a Canon 2X and although it can produce decent images with the EF 300 and EF 400 f2.8 glass, the images are clearly impaired compared to what these extremely sharp lenses can produce by themselves. The results with a 2X will be far worse on lesser glass and I would avoid the 2X. Also, a lot of camera body/lens combinations won't autofocus with a 2X extender.
Realistically, if you feel you are going to be using the extender for a very large percentage of your photos then you need to compare the extender/lens combination to the results from one of the 150-600 setups.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
