This is by far Canon's most affordable 400mm prime lens that is L quality. Compared to Nikon, if you wish to reach 400mm, you have no choice but to purchase the ultra expensive 400mm/2.8 or use third party lenses.
So in effect, Canon users are lucky we have three 400mm primes -with the 400/5.6L being very affordable. Image qulaity is superb, but what I like most is the godly AF speed. For this reason, I very much prefer this lens to the 300/4L IS with a 1.4X TC - which many people have agonised over
While the lack of IS is sorely missed, don't forget that Nikonians have mostly been shooting telephotos without IS or VR and they are doing fine - so I don't find the lack of IS such a big issue especially since the 400/5.6 is priced so affordably