Roger Cicala over at Lens rentals has had a chance to test the new 100-400 he says ...
"From a resolutions standpoint I wouldn't rush out to upgrade my old 100-400
IS L. The resolution difference is mostly in the corners at 400mm, and most of us are more interested in center and mid-field sharpness at that focal length."
http://www.lensrentals.com/blog/2014/12/a-brief-400mm-comparison
Good news - unless you want the better
IS and ring zoom, you don't need to rush out and replace the old version. It's not going to get you much in the way of improved
IQ. Also, if you don't have one at all, you may find a 2nd hand bargain of the old version and not lose out on quality.
Bad news - it hasn't been improved as much as might have been hoped. I wonder how this will play out on the now (almost) confirmed 50mp sensors coming this year?
Cheers,
Colin