I've had this lens for awhile, but only recently had a chance to really put it to the test, having spent 3 weeks in Costa Rica. I also own the 100-300mm and while it is fantastic at the short end, it's not good enough for serious work at the long end. The 100-400 reverses that.
Durable; Serious glass; with great IQ at the long end
Easily becomes detached from body; Unimpressive at 100mm