I bought this lens after read the previous reviews to complement my, in the way, 24-105 IS.
After my firsts shots iīm really impressed with the lens. Itīs as sharp as my 300/4 IS and very far from my 70-200 IS. I really think itīs my best buy in Canon lenses. I still prefer the the 300/4 plus 1,4X for focal 400, itīs more sharp, but the 100-400 is much more versatile.
I own a 135/2, 14-70/4, 24-70/2.8, 70-200 2.8 IS, 300/4 IS...
and i think the 100-400 itīs my best lens.
Please, excuse my bad english.