comparison between 100-400 and this lens in my opinion is not right... we are comparing prime with telephoto .. this is way quick and perfect for BIF shots.. i suppose this lens is made for that.. 100-400 AF really hunts and i have seen it in action as one of my friend owns it.. i was really not happy and my interest mainly is BIF shots and for perched shots 100-400 is real good but even for that 400 5.6 is sufficient.. just up your shutter by more than 1/500 and there u go with crisp sharp photo...
.. use it to believe it.... if ever i own a 500mm F4 or 600mm i am not going to trade this lens...................NO .. even then , i might not sell this lens coz nothing can beat this lens for action shots even 500mm or 600 coz we cant handhold them for long.. this is like a piece of stick in hand and we can hardly feel any trouble even after holding it for couple of hours on the field.. my rating for this lens is 5/5 .. no second thoughts.. buy it and live happily ever after