The 'old' 100-300mm f5.6 and 100-300mm f5.6 L were 'decent' for their day, with the L receiving good compliments for raw optical quality. However the focus (slow, noisy) was often complained about. Their size meant that it was handy as a travel zoom also. I think the current 100-400mm L is quite different in most aspects to the above two lenses, though certainly some optical aspects could have been carried across (the 100-400 having variable aperture, IS, 100mm extra, much longer lens housing, etc.)
The 100-300mm f4.5-5.6 USM improved the focus on both 100-300mm f5.6 lenses, but sadly is lacking in IS - and the optical quality at the long end left more to be desired, especially wide open (not very sharp, and very low contrast). I bought this lens (my first 'tele zoom' lens) - and knowing it's limitations, I was able to get a lot of good photos (eg stopping down between f8 and f11, and using some clever post processing). It has great AF (fast and accurate), good build quality and is of a convenient size.
When I stepped up to the 70-300mm f4-5.6 L USM IS there was an improvement in every area over the 100-300mm f4.5-5.6 USM (except
for size, weight and price)! That is:
- the usefulness of the IS can't be overstated for many applications (eg wildlife)
- the AF is even quicker and I'd say a touch more consistent / accurate
- build quality is 'tank like' (solid, super smooth focus and zoom rings, and FWIW I prefer the focus ring closer to the camera body, and the zoom further away - I know some other people differ)
- much reduced CA (the 100-300mm had lots of noticeable purple CA at the tele end)
- faster aperture and retained longer in the zoom range (it's almost like a 70-200mm f4 IS with an extra 100mm)
- 30mm more zoom range on the wide end
- (and this is the most important one to me) - a huge increase in optical quality at 300mm f5.6, where from corner to corner its super sharp and has lots of contrast.
I hope that any replacement to the Canon 100-400mm L would take the above good qualities of the Canon 70-300mm L, and apply them - to be a popular lens for many photographers. To me, it's going to be 'too big' as a tele-zoom travel lens, and not really long enough for the 'super serious' birding. The alleged price (of $2800 USD) is a considerable jump from the $1600 for the 70-300mm L.... Certainly though, many people will be prepared to pay for it, if it delivers the goods (look at how popular the 70-200mm f2.8 II is!)