I'm considering purchasing one of the Canon 1.4X or 2X extenders, for use primarily with a 5D Mk2 / 70-200F4L camera and lens combination, and wondered if anyone can comment from experience on the image quality you can expect to get from them, particularly how big a difference there is between the II and III versions of them.
I use the 1.4x TC III with my Canon 70-200 f/2.8L IS II and Canon 500 f/4L IS with Canon 1Ds mkIII and for both the TC III is really worth getting over the old model which I have sold. It's not only sharper in the corners but over all the image area with better contrast and details. I have a 2x TC II and tried the 2x TC III on my 500mm lens but I would not invest the money in this extender. I have made a few test shots on the 70-200 f/2.8L IS II with the 2x TC II. They are surprisingly sharp and certainly better than upsampling from 1.4x TC III which I have tested too.
The problem I found with the 2X TC (both II and III) with my 500mm lens is the bokeh which does not have the butter smooth appeal as without the TC at all. With the 1.4x TC the bokeh can suffer sometimes especially for wild life with shiny straw or branches behind a bird or animal. I have not done an extensive test per se of the bokeh between the different TC's and on different lenses, I have just noticed browsing through the images I have and taken with the different TC's that the 1.4x TC III stands out and the 2X TC's are nice to have, but really not good enough for critical work in most cases. As mentioned it is the bokeh that really suffers with the 2x TC's more than sharpness and details. Contrast also suffers. The 1.4x TC III can give the crispy look (almost) that you get from a naked lens like the 70-200 f/2.8L IS II (and the Canon 70-200 f/4L IS) and the Canon 500 f/4L IS even fully open.