I rented a Canon 400mm f/2.8L IS USM lens 8/14/09-8/17/09 at Pictureline (300 West 700 South, SLC, UT or pictureline.com) and can't remember when I've had as much fun: my KLR 650 might find itself replaced by a lens.. This lens is a Canon masterpiece imho.
I picked it up at 6:30pm and went out for Chinese. Back home, I thought it might be too heavy for my Wimberley sidekick. By 8:30pm I was convinced otherwise but the sun had set. I took a few shots in my yard in the dark.
The next morning, 6am-7am, I took more shots, before sunrise. At 7am clouds rolled in and I scrambled to avoid the thunderstorm. By nooon the sky was clear, company wasn't here yet, so I took the lens across the street and spotted a few more coyote AND a redtail hawk circling overhead. More shots. I took the lens into the park Sunday and Monday for another 12 hours of use before returning it. One more gallery needed for the lens, "Lucky", because I saw/captured a good cross section of local elusives..
The lens is huge/heavy but it's so sharp: heavy lenses need love too.. (I tried to trade pictureline my 100/400 for the 400: more versatile, lighter, smaller, same cool white color. They weren't interested..)