I took this lens, mounted on a 5DSR, on a flyfishing trip to Iceland last week. Even though the reach is not what I normally use, the combination with the enormous resolution of the 5DSR gives the combo a fantastic flexibility.
This dunlin, which is a rather small bird, came quite close. You do not get 400 f2.8L IS II type bokeh, but I can live with this.
5DSR, 1/1000s, f/5.6, ISO320, @400mm