Canon’s 100-400 maybe isn’t the best-built lens, the push pull design makes the bearings and zoom lock prone to picking up sand or dust from the lens’s body. However, it is one of my favorites. I actually like the push pull zoom for a lens of the range, and my copy is very sharp.
However, for me the biggest advantage of the 100-400 is that it’s nearly identical in size to a 70-200 f/2.8. That makes it easy for me to size my bags and pack my gear; need speed take a 70-200, need reach take a 100-400.
Unfortunately, the 100-400 has a well-known tendency to suffer from failures in the zoom mechanism.
This is just what happened to me this morning at the BirdPhotographers.net meet up. Kicking the morning off and everything is running great; the lens is smooth as butter. Walk around a couple of times, and go to adjust the zoom position and bam, zoom won’t slide, zoom lock ring is stuck, and the focus ring is stiff and feels like there’s grit in it.
Hopefully the fix won’t be as expensive as I fear it will be.