I, for one, am very excited to add this lens to my kit. It more-or-less replicates Nikon's AF-S Nikokor 80-400mm f/4.5-5.6 lens, which Thom Hogan http://bythom.com uses on his photo safaris and workshops to produce absolutely beautiful images. Physics and cost constrain Fuji to the same f-stop range that Nikon faced. With airline size and weight limits getting ever more stringent, an f/4.0 lens is not w elcome prospect. Fuji X-Series high ISO performance is so stellar that getting that extra one-third to one full stop of exposure does not seem like that big of a sacrifice to me.
Like Lichtlicker, however, I am astonished at the 82mm filter thread size. A large objective lens is a major contributor to lens weight, along with the added weight of the larger diameter lens body to accomodate that larger lens element. I'm hoping the filter size shown is in error, and that it will come with come with 77mm threads. For what it's worth, Art Wolfe keeps a polarizer on all of his long glass to get more saturated colors, and you can imagine the cost of an 82mm circular polarizer....
Regarding the size comparison with the XF 50-140mm f/2.8 lens, note that the XF 100-400mm lens in the photo is sitting on the rear lens mount, and not on a lens cap, which would add even more to the height/length.
Just my random thoughts. I used the 55-200mm lens in the Galapagos Islands last November, and I was VERY disappointed by the extremely poor autofocus performance and the short reach. Personally, I can't wait to get my hands on this lens.
