.... Are you thinking of it as a step backwards for me because I already have the Olympus ED 300mm F/4 Pro?
I photographed some of my surfer friends this afternoon and because of the rising tide and sea conditions they were at times very far out at sea. So instead of mainly using my Olympus 40-150mm + MC-20 I mostly shot on my 300mm + MC-14 which combo gave me more reach. But as anyone rode a wave in towards the shore I needed to pick up my other body with the shorter reach zoom lens mounted. Due to the strong winds it was much easier shooting from my tripod. Now if I had the 100-400mm today I would have had the flexibility and not have missed so many shots.
When shooting wildlife this year there have been several times when I have wanted the flexibility of zoom when using my 300mm prime and have been restricted by it.
Even when I bought the 300mm it was planned as only temporary until the 150-400mm+1.25x was available but now I know more detail about that forthcoming big Pro lens I feel that the 100-400mm will be more practical.
If someone is bothered to make scientific comparisons between the Olympus 300mm and 100-400mm I am sure they will conclude that the 300mm is a 'better' lens but everything is indicating that the 100-400mm is absolutely good enough for my purposes.
Also, contrary to RED35's review below that if you already have the 40-150mm plus Teleconverters (which I do) then you don't need the 100-400mm I absolutely do not agree and I most definitely have a use for both.
It keeps coming back to the same thing and that is simply that each of us have differing lens requirements and preferences.
View: https://www.youtube.com/watch?v=TIcH1U4vkis