This lens, as with all Canon L series lenses is built for professional use, so quality of construction is excellent. What is unusual nowadays is that the Canon 100-400L is a push/ pull zoom, which although it functions just as well, is a little confusing used after a bag full of two ring zooms. Although there are lots of 70-300 lenses around, this is in a different league in terms of performance but also in size and weight. Most lenses of this sort of spec exist so that you don’t have to carry big heavy lenses around - with the Canon EF 100-400L it is the big heavy lens at 1380g and 189mm long. So why should I want one?
The answer has to be because it fills the same niche of covering a large range but it’s performance has none of the compromises your average consumer 70-300mm lens comes with - usually a poor performance at the longer end of the zoom range. Also, with the Canon Image Stabilisation system built in, the slower than usual pro spec maximum apertures are less of an issue - in theory at least turning this into a 100-400mm f 2 - 2.8. Now when you put it like that the Canon 100-400mm f4.5-5.6L starts to sound worth it’s rather high price tag.
After testing the superb Canon 300mm f4 IS L the Canon 100-400L test shots, although very good through the range, aren’t up to the standard of the fixed focal length 300mm. What is better with the 100-400 IS is the image stabilization system. Some of my shots at the long end of the zoom shot at 1/50 were sharp enough to be useable so the gain is, as Canon claim, at least two stops. Judged as a package this is a very useful lens, but like almost all zoom lenses, it’s a compromise.
To be honest, judged purely on usability, ignoring cost, I’d rather have the excellent Canon 70-200mm f2.8 IS and a convertor - that way, at least in the 70-200 range, I’d have the fast aperture, and the image stabiliser, and performance comparable to the best fixed focal length lenses.