The 100-400L is the lowest of the hi-end L's. As such it is ok, good colours, good sharpness and bokeh too (take a look at fredmiranda.com).
BUT the focus motor is SLOWSLOWSLOW, which makes it a bad choice for action photo. Which is me :(
Unfortunately, Canon doesn't offer any alternatives in the zoom line at 400 (there's the 70-200 f2,8 IS, which is great, but only 200) - Otherwise, you'll have to go for a prime.
Next choice would be a non-Canon lens; If you need stabilisation, Sigma has a 80-400OS that should be ok (haven't tried it), otherwise there's always the Sigma BigMa (50-500), which is very popular (haven't tried that either)
Perhaps you would get back to me when you have decided - I'm eager to know of possible alternatives to the 100-400L..