I only shot once with the 100-400, and it is a wonderful lens. Same w/ the 70-200.
The thing I will say though, is that I prefer the 70-200 for general shots over the 100-400 for obvious reasons.
The 70-200 is great for the closer in shots as per the fact the 100-400 will DEFINATLY reach out. its not a lightweight lens, nor is the 70-200, but it is fantastic when out in the sticks shootin critters.
However, because I try to shoot critters from several dozen yards away, the 100-400 would be a nice addition to my collection. Until I can afford it, I will use the 400mm sigma.