I find the auto ISO performance of the 1DX to be amongst its most usable features. When I do wildlife, birds and macro shots, I want to have full control over both shutter speed and aperture and therefore primarily use manual mode. Avaliable lighting can shift dramatically and by using any of the program modes (P, Av, Tv), you only control either shutter speed or aperture and not both. With fast moving objects and long lenses, you first need to find whatever you are shooting in the viewer, than try to frame it, then get the focusing right and on top of this adjust ISO settings by controlling shutter and aperture. Puuhhh
On earlier bodies (1DsIII, 5DII etc.) I actually never used it, to secure the lowest possible ISO settings. But many shots were ruined by slow shutter speed (Av mode) or too deep or too shallow DOF (Tv mode). With the high ISO performance on the 1DX, that is much less of a worry and I rather accept occational grainyness, with proper DOF and shutter speed, than the alternative.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1