What are current satisfied 100-400 owners thinking?
I don't really see any faults with mine (not to be confused with knowing there's room to improve) so I'm in two minds lol.
My 100-400L has the same faults as the other four copies I tests.
First, it's got lousy handling. The lock ring is the reasonable place to put my hand, and that means zooming causes AF changes if you aren't really careful because turning the lock ring turns the AF ring with it.
Second, the IS is so bad, it's nearly useless. It can even be worse than useless (see below).
Third, from 300-400mm wide-open with the IS on, the position of the IS elements greatly affects the results. The results can be great if the elements happen to be in the middle, or horrid if they aren't. In practice, this means 300-400mm with IS means f/8, and since the IS is good for *maybe* 1 stop, it's no better than f/5.6 with the IS off (for which the optics are quite good).
I suspect they fixed all this nonsense in the new version.