I shoot aircraft and did find the zoom helped. But When handholding, it is a lump to pan and try to zoom and keep steady. I'm used to heavy lenses, but at times I found it a bit of a hassle handheld when panning and zooming together.
I had the 200-400mm from new, sold it, bought it back fom the same guy and now sold it again.
It's that kind of lens. I loved it. Thought I would use it more both times, but tended to use the 300mm 99% of the time. Purely because of the results I was getting from the prime.
Kev.