I must confess that I don't own many L lenses (only have 4) but the 100-400mm, being a push-pull zoom lens, has some serious flaws for extreme use (which is what L lenses are designed for anyway). On a recent trip to the subantarctic, a large number of photographers with the 100-400 L (and the other push-pull zoom 28-300mm L) had lens failure, either intermittent or permament. The most common problem was the AF being jammed (cannot manually focus either... hence it was the focus ring being jammed). The second most common problem was most likely an IS problem, with the lens giving an Err99 whenever the shutter was depressed with the IS on. There are other problems from the inability detect the lens by the camera, to inability to change aperature, to even having the zoom being jammed. I must specify again that this is under rather extreme conditions (temps above 0 but less than 10, and with constant sea spray and high saline saturation in the air. This experience has been repeated quite a number of time with other photographers who use the lens on less extreme pelagics (out in the sea). In short, the push-pull design zoom lenses are not suitable for use around marine environments. I did not experience similar problems with ring zoom designs.