Love your picture. I've been thinking of getting a 100-400 for my 7D. How do you like yours?..
I really like it, and there's nothing else out there like it. I'd love to find something in this range that's affordable to try on my Nikon gear but so far I don't see that option.
It's not TOO heavy, the slide zoom takes a bit to get used to but once you do it's very quick for things like running animals and even birds and bugs in flight; I could use more practice.
I was getting some decent images from it with my older bodies but it didn't really shine for faster work until I put it on a 7D. After I got used to the AF system, or at least got better with it, it's been quite amazing with that lens. I've been getting bang-on AF using servo tracking for small subjects using spot-AF. Not easy to keep single spot AF point on target but I've found the assist/group AF doesn't tend to work as well for my shooting with busy backgrounds. But that's just me and the 7D..
AF performance on a 5D3 or 1Dx should be really good. On 40D, 60D and even 350D it worked quite well too, just not up to fast tracking like the 7D (or other fast bodies.)
The lens is really sharp up to 200mm then gradually softens towards 400mm. Not much, mind you, it's still quite decent and I'm often shooting with it at the long end.
Bokeh can be rough in transition zones with fine detail like grass or branches but acceptable otherwise.
CA isn't bad and easily corrected.
The IS could be improved for handheld static subjects but at least it helps. I'm usually panning with it so IS is of limited value. Surprisingly, with birds in brush or clutter I'm often manually focusing too with decent results.
It's definitely great for bird close-ups at typical backyard distances, wildlife, even some kinds of landscape shots. I tried a few long range shots with a 40D and 1.4 extender on it but the AF missed a bit and subject (running fox in evening light) was not in sharp focus. Longer range work tends to require fast and pricey 600mm glass.
OTOH, some say the 70-200 f/2.8 L IS II with a 1.4x or 2x extender (latest versions) works pretty well too if you already have that lens. I haven't tried it to compare since I only have the 1.4x v2 and already had the 100-400mm.
So, it's not an ideal lens, but it covers an extremely
useful range and performs pretty well optically and AF.
If you can find a used one in good shape and good price I think you might enjoy what it can do for providing a new perspective.