More accurately.. not the best Canon lens but one of the best zooms. There are better lenses and each lens has advantages versus disadvantages. The push pull design of the 100-400mm does take getting used to and is very much a personal preference. Never know if you don't try it... I personally found it faster to go from the shortest focal length to the longest focal length than twisting.
In daylight outdoors, at a polo match. My cousin had my 70-200 f2.8L IS on a film camera and I had my 100-400mm on my digital. By far, the 200mm focal length was short. For most outdoor lighted events, the 400mm focal length will be more than welcome. (and when I say lighted... I'm not saying blinding bright light.. just good quality light) If I had to choose between bumping up to ISO400 and getting an up close shot versus shooting ISO100 and a wide shot, guess which I would choose.... Too many people are sold on the myth that a zoom that doesn't have a max aperture of f/2.8 is useless.
Now if your are talking indoors sports (smaller arenas), night time events, weddings and such.. 70-200mm f2.8 all the way. Actually... I'd probably go with a couple bodies, one with a medium zoom and another with a long prime.