[This review is of the 1998 version of this lens. A new model has since been issued by Canon.]
Ding ding ding. This is it. THE most fun I've ever had with a rental.
Start lifting weights. I remember taking this lens out for a day at Chincoteague, and it got heavier as the day wore on. This is a BIG lens. Canon lists it at 4 pounds, which is easily twice what I was used to carrying. The results are worth it and you can plan an entire day around this lens.
Coming from my own 70-300 lens, the photos from the 100-400 were a revelation in their clarity and detail. And that extra 100mm adds much needed reach for birding. In the photo samples at the bottom of this page, all of the birds in the dead tree are at least 100 feet away, yet this lens gave me blue jays and mourning doves in beautiful clarity.
The lens dwarfs my old Rebel XT. On the barrel are image stabilization switches, auto/manual focus switch, and a switch to limit the focus range for faster auto focus.
This lens uses a push-me/pull-you zoom action, where you actually pull the lens away from the camera, extending its reach. "Tromboning."
Owl visited my tree at dusk, not the best time for photos, but a good chance to test the range of this lens.
Both photos (above and below) are at f/5.6, 1/60, ISO400. Above at 160mm, below at 400mm.
Another sample of the zoom range. (Above: f/5.6, 1/1250, ISO100, 100mm; Below: f/5.6, 1/1000, ISO100, 400mm)
Following are a few of my earliest photos with this lens, from back in 2007
f/5.6, 1/1600, ISO200, 400mm
f/5.6, 1/250, ISO200, 400mm (at about 80 feet)
f/11, 1/160, ISO100, 400mm at about 100 feet