The Canon EF 100-400mm f/4.5-5.6L IS II is a remarkable lens. I should preface this review by stipulating that I, as a rule, am not a fan of zoom lenses. The vast majority of my lenses are prime lenses. I do not own any Canon 24-70mm or Canon 70-200mm lenses.
The combination of this lens as well as the Canon EF 16-35mm f/4.0L IS is enough to restore my confidence in zoom lenses after less than satisfactory experiences with the dreadful Canon EF 24-105mm f/4.0L IS and Canon EF 17-40mm f/4.0L.
The 100-400mm is a large lens bordering on unwieldy (depending on camera body). It is also almost unusable for night sports. None of that matters however, as this lens is perfect for sports photography. I've owned the Canon EF 300mm f/2.8L IS and this lens is so much more versatile than virtually all other offerings from Canon. In most sporting events it is a well known fact that the competitors will not remain at the same distance from you during play. It's all well and good if you have a fast 300mm or 400mm prime handy if your subject is the right distance from you, however, this rarely happens.
Assuming you are shooting a sport in the morning or afternoon this lens means that you will rarely have the wrong crop for your photo. You simply have to readjust your focal length over this large focal range to suit your desired crop.
The autofocus on this lens is blazingly fast on the 5D mkIV and rarely, if ever, hunts for focus. Despite having a narrow aperture the bokeh is quite pleasing. Obviously between 300mm and 400mm there is such a compression of the background that subjects have glorious three-dimensional definition on your photographs.
Further the minimum focus distance is impressive and puts lenses like my Canon EF 85mm f/1.4L IS to shame. If you can deal with the weight of this lens then this truly is a special offering.