How, lot's of strange comments in this thread regarding the 100-400L. I own both the 70-200 f/4 IS and 100-400L, and they are very different lenses for very different purposes (at least for me). Anyone who thinks there is not a big difference between 200mm and 400mm obviously doesn't shoot with a longer telephoto much. I find 400mm the minimum focal length essential for serious wildlife shooting, and the 100-400L provides excellent value with it's combination of versatility, image quality and auto focus performance.
While f/5.6 has some limitations, with the high ISO performance of today's newer camera bodies it is becoming less of an issue. With the 7D, I can push ISO to 6400 and still get acceptable results with proper exposure and a little noise reduction in post. With the 5D3, ISO 12,800 is not an issue.
The 100-400L gives me sharp results when shooting wide open at 400mm. I do need to keep my shutter speed up when shooting handheld, but that limitation is a decent trade off for the image quality. Besides, I will quite often use a monopod when using a tripod is either inconvenient or not allowed (such as at the zoo).