At first glance, this L-Series Canon lens seems to be a bit less distinguished than some of its telephoto brethren. It doesn't have image stabilization, and at f/5.6 for a maximum aperture it's a bit slower than some alternatives, better suited to daylight in the outdoors. But you'd be so wrong to judge it without taking a second look.
If you're a nature or news photographer, we defy you to get better, sharper image quality across the aperture range out of a lens in this class. Canon makes some terrific zoom lenses that cover this range, and while they'll deliver great shots time and again at this focal length, if you're shooting with a pro camera body with a high pixel count, you're going to appreciate the optical resolution this lens deliver to your sensor. Plus, it's lightweight for its class and reach, thanks to the fewer glass elements necessary for its design.
The glass that is there is of impeccable quality, as you'd expect from a Canon L series lens. One super-ultra-low dispersion (SUD) element and one UD element ensure a minimum of color fringing. That means every nuance of color and texture on the wings of that bird on the tree limb across the field will be clearly visible and reliably recorded.