3 day rental (pictureline, 300W 700S, SLC, UT) of the just as sharp but much easier to tote Canon EF 300mm f/2.8L IS USM. The lens has half the weight, half the rental fee, half the price, but all the sharpness, aperture, and function of the 400/2.8. It just doesn't have all the reach..
This lens, w a 1.4x TC and camera attached, fits in the pack. It's very nice for wildlife photography to which one must hike..