A friend of mine has that lens and a EF 70-200mm f/2.8L IS USM and for most of our photo meets he brings the EF 100-400mm f/4.5-5.6L IS USM. Not sure why he prefers it though...never asked. He might just like the extra range the 400mm end gives him.
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.
70-200 f/2.8L
70-200 f/2.8L IS
70-200 f/4L
70-200 f/4L IS
All are considered high quality excellent performers. The 70-200 f/4L versions are proven sharp even wide open and very compact. The 70-200 f/2.8L versions are a popular choice for wedding and people who require zooms in low-light. This is a fairly popular focal range and it is nice Canon has so many options. I wouldn't hesitate to recommend any of them....
Just like Samanax's friend, I chose the 100-400L over the 70-200 f/2.8L because sheer flexibiilty of the long focal range for the weight and packaging. At times, i do miss my 70-200 f/2.8L IS.
I'd say the 70-200L is still slightly sharper than the 100-400L at f/4.6-5.6 etc. This is because the 100-400L is shooting at wide open and most lenses are not optimal wide open.
Then again..
The 70-200L is incapable of shooting past 200mm...
For wildlife, you'll want the extra focal length. Ideally long telephoto primes.
I guess I just want a zoom lens to add to the collection. I was thinking between the 70-200 f/4 (wouldn't need the 2.8 for majority of the shots outdoors) and save some dollars or go with the 100-400L for the longer range.
So all in all you like your 100-400L then? Seems like it has some good reviews on it. I think the range on it would be great for nature shots.