I rented the Canon 200-400 for an air show. All were propeller planes, so I shot at slow shutter speeds to capture prop blur. All hand held. This is not a lens test, just some shots to show how the lens worked for me. Keep in mind that panning at slow shutter speeds takes some skill. I'm sure others could do better than I can.
I like sharp images, so much that I call myself a sharpness freak
I own the 300 f2.8 IS II and 1.4xIII, which is one of the sharpest lenses available. So I had some thing to compare the 200-400 to. The results I got with the 200-400, even with the 1.4x engaged, are very satisfying. The sharpness and detail are very good, approaching my 300 f2.8 & 1.4x combo.
Here's a few shots. They look sharper when viewed from my pc. I lose some sharpness and detail when I upload to the net. If anyone is interested, I could provide a few raw samples on dropbox.
560mm @ 1/250s
560mm @ 1/180s
250mm @ 1/160s
350mm @ 1/160s