The 14-24mm f/2.8 is an absolute joke especially compared to the 14-24mm f/2.8 from nikon or if you are willing to sacrifice fov the 17mm TS-E is so much better. I would pick any of those lenses before I went with Canon's one.
Are you from the future commenting about a lens not released by Canon as on date? If yes, please let us know on what date the production model of the 200-400 was released ... I'm tired of waiting
5D3, 6D, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 600RT x 4
I come here to learn something new, not to learn how bad my gear is - I know that already ;-)!