I haven't owned either the original Canon 100-400 or the new one, but my good friend has it and I have the Sigma 100-400. I use it mostly for action shots (of my dog, mountain biking, etc) and landscapes. To me, it's very sharp and focuses quite well. One of the supposed negatives was the lack of focus performance on objects that are coming straight at the lens. So I wanted to test that out, since I plan on shooting some MTB action shots with the rider coming right at me.
I've taken several others, but here's a shot I took this afternoon of my 2-yr old chocolate lab running full speed right at me, chasing her ball. I'm not sure how it'll upload here, but the picture is tack sharp when I open the file in LR. Taken with the 5D4.
It's less money and much lighter, so to me it's a no-brainer since it's one of my lesser used lenses. But it's very well made and the performance is right up there with my other lenses.
please log in to view hosted photos in full size.