I’ve been using Canon’s f/4.5-5.6 L USM IS lens since June and I must say I’m very happy with it. It certainly takes time to learn since I have no super-telephoto experience. It’s heavy; with the camera it weights about 5 pounds. Nevertheless, even with my slender build, a 3 or 4 hour foray into the field doesn’t seem to pose neck strain, though tripod breaks definitely help.
Although the USM autofocus is very quick, it’s not so effective when photographing birds where there are often leaves or branches in front of or behind the subject, often resulting in an out-of-focus bird. Manual focus seems to be the better alternative under these conditions. When extended to greater magnification, the image stabilization feature helps but you really need to brace the camera against something solid if you’re shooting at less than 1/800 of a second, otherwise the image will not be sharp. As always, it’s important to know your subject so you can be in position with the necessary camera settings to get the best shots. (More photos at: http://sfielding.photoshop.com)