Well, I think the review ( http://www.bobatkins.com/photography/reviews/ef_100_400_l_is_review.html ) says it all. If you want a zoom that goes out to 400mm, the 100-400 is the best.

When it comes to the 70-300 vs. the 100-400, the 70-300 is smaller, lighter and cheaper and holds it's own pretty well in terms of image quality, but it is really limited to 300mm. It won't take a Canon TC and results with a Tamron 1.4x, while OK, are only just OK. The 100-400 is very sharp at 400mm and still pretty good at 560mm with a 1.4x TC attached.

If I didn't already have a 300/4 and a 500/4.5 I'd give serious thought to the 100-400IS. As it is I'm keeping my 70-300 because it's small enough and light enough to carry around easily and I have other lenses if I need more reach.