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Wow, nice images displayed here ! Indeed, that lens seems to be a great performer. As light and compact as the 70-200mm f/4.0 IS but with more reach, but slower and more expensive. As fast as the 100-400mm, but lighter and more compact but less reach. A great compromise between the 70-200mm f/4.0 IS and the 100-400mm, I guess. For sure, Canon's offering in this range doesn't fall short.
I have the F4IS and considered the 100-400L when I bought the 70-300L. I went with the 70-300 because it was about 2/3 the size of the larger, longer lens, and it also had better IS and the latest coatings. I was also concerned the 100-400L might be replaced, but that wasn't really too much of a factor. I could use 400mm at times though. I guess there are always compromises. I went for the more compact package for travel.
If you don't need the extra reach the 70-200 F4IS is a fantastic lens. However I was adding a 1.4TC often in windy, dusty conditions and that made me look for a longer native alternative.