This lens needed a replacement far worse than the 24-70 or 70-200.
I used mine on Friday and the IS system just flat stinks compared to even the old version of the 70-200. I lost about 90% of the shots I would have gotten with my 70-200 at the same focal lengths and shutter speeds.
I don't mind the push-pull, but the handling of the current 100-400 is the pits. The most comfortable place to put your hand is on the locking ring and focus ring meaning your constantly messing up focus while zooming. The focus ring should be on the very end like it is on the 70-200, and the locking ring shouldn't move the focus ring.
That is not my experience now. Before AFMA though, I was very unhappy with my shots using the 100-400. However, I shot 400 odd pictures on Saturday most of a hawk while doing AFMA. I shot about 200 after I was happy with the AFMA and all were hand shot with my 100-400 on the 5D3. All those came out sharp. I put up a few images yesterday that you can check. The link is http://www.canonrumors.com/forum/index.php?topic=7576.msg138751#msg138751.
I hold the lens under the tripod mount as I feel that is the most balanced location for me.
This had nothing to do with AF, it had to do with IS being pretty darned worthless on this lens, at least compared to more modern lenses.