I'm thinking that enough of us arent doing our homework to realize that many of the lenses wont work with either 1.4x or 2x converters on them unless they are f/2.8 or lower.
I know I've bought two new converters in the past couple of weeks both from people selling them because they said they bought them and didnt know they wouldnt be able to keep their autofocus with the converter on.
With our lenses, most are f/2.8 models, we can seem to get tack sharp images with the extenders on the lenses. Granted, I'm following most of the rules and having the lenses mounted on a sturdy tripod with a Wimberly head to hold the lens steady, and have Image Stabilization on the lenses, but before you buy anything, make sure you do a little research.
Check out some of Arthur Morris's work
if you think converters/extenders dont work. Many of his images are with stacked converters, yes, thats a 1.4x on top of a 2x and they're on an 800mm lens!!!!
This guy is one of the top Bird photographers in the world, so he obviously knows what he's doing.
Most of my bear and wolf shots I recently posted in the Wildlife thread are taken with a 400mm and a 1.4x converter and the lens was only on a monopod. But it was on a Wimberly also, which makes up for 90% of my mistakes....LOL