My wife and I do a lot of hiking in venues like national parks, and I typically carry two bodies, one with a wide/normal zoom (17-40 or 24-105) and the other with a telephoto zoom (70-200/2.8IS), along with a 1.4xII.
The 70-200's fixed length takes up a lot of space in the backpack, and mounting/removing the 1.4x as needed is cumbersome and time-consuming, and only adds to the length of the rig.
I would buy a 70-300L today to replace the 70-200/1.4x combo for this application (I would keep the two optics for other purposes), but the "reversed" placement of its zoom and focus rings puts me off.
For the life of me, I can't understand why Canon chose to design it this way. Weird. There may have been a technical reason, but couldn't they have figured out a way around it? They might as well have reversed the action of the zoom ring, while they were at it. I know that if I were to buy one, I would constantly be reaching for the focus ring when I wanted to zoom.
This is also the first Canon white (to my knowledge) that doesn't include the tripod ring with the lens. Again, weird.
Anyway, I'm going to wait a while longer, and see if Canon releases an updated 100-400. then I'll compare that to the 70-300L, and pull the trigger on one of them.