Replaced both my 70-200 F/4L IS and 100-400L with this lens. It is reasonably sized, built like a tank, sharp wide open, fast focussing, and has great IS. IQ is indistinguishable from the 70-200 F/4 IS which for a long time was rated as Canon's sharpest zoom.
Only downside is that the tight gits at Canon decided not to include a tripod collar. I got a 3rd party one from Amazon for £30 which fits and matches perfectly.