I've owned the 100-400mm, 300mm 2.8, 300mm 4.0, and 500mm 4.0. Here's my take,
1. Firstly, Canon lenses are variable in quality, and that's true of even the most expensive L lenses. Just to give you one example, I had to return my new 500mm 4.0 for optical correction due to de-centering, the image was consistently slightly softer in one quadrant than in the opposte quadrant. This quality variability can be quite marked and it means that in practise you have to look at the performance of individual lenses rather than at the lens's MTF chart. It's a shame, the MTF charts for Zeiss and Leica lenses tells you pretty much what you're going to get, the MTF charts for Canon lenses tells you the best you can hope for, if you're lucky.
2. Some of the lenses you're discussing are starting to get a little bit long in the tooth, for example most only have 2 stops of IS. There's been two generations of IS development since then, which can make quite a difference in practical image quality terms. Don't forget, many of the best wildlife shots are fired off handheld in extreme urgency.
3. My 500mm 4.0 was almost identical to my 300mm 2.8 in the centre of the frame, but the 500mm had the advantage at the frame's edge. The fact is Canon's multiplier's are "jack-of-all-trade" optics rather than performance matched to one particular lens, and it shows. For comparison I also use the Hasselblad 350mm super-achromat, it comes with a dedicated x1.4 multiplier, comparing performance with this as against the standard Hasselblad x1.4 multiplier is like comparing chalk with cheese. As further evidence I'd cite the Canon Lens book which gave MTF charts for most lenses with and without the x1.4 and x2.0 multipliers, it's clear that these multipliers have variable performance with different lenses.
4. I've subsequently sold my 300 2.8 and replaced it with a 300mm 4.0, which is a superb optic and one of my all time favourite Canon lenses.
5. One of the most important Canon developments is happening right now with their RAW converter, from the converter scheduled for release later in 2007 it will start to correct the image for several optical defects, including colour fringing, vignetting, and distortion. It's my guess that we'll see much more development in this direction with significant implications for lens design. But here's the rub, many current lenses can't take full advantage of this new technology. Paradoxically some quite old lenses (like the 100-400 and the original 85 1.2) can, while some more recent lenses (like almost all of the longer lenses) can't.
6. I sold my 100-400 after trawling through three versions to find a good one. I found the ergonomics poor and the image quality very variable, not only across the zoom range but also in terms of far/near focus and centre/edge consistency. When it was good it was excellent, but most of the time it was just indifferent.
Just my opinion.
