I bought the Canon 100-400mm IS L-Series lens when I bought my Canon EOS 3e film camera back in the 1990′s. It has gone just about everywhere with me. I always keep it in my daypack, along with a camera body and the Canon 24-70mm f/2.8 lens. When I can’t take the Canon 600mm prime lens with me, the 100-400mm is my long lens for bird and wildlife photography. It’s much easier to travel by plane with this lens than it would with the 600mm lens. When mounted on a camera body with a smaller sensor, such as the Canon 20D or the newer Canon 50D, the “multiplication factor” of the smaller sensor makes the lens equivalent to a 160-640mm lens on a body with a full-size sensor, like my old EOS 3e. And the image stabilization makes it possible to hand hold some shots that might not otherwise be possible with a non-stabilized lens.
This is an L-Series lens, which is Canon’s family of higher end lenses, but reports I’ve seen, and my own results, show that it is not as sharp as other L-Series lenses. However, on a day with good light, it does take great photos that can be printed at 13×19-inches or larger. And it definitely takes much higher quality images than the Canon 75-300mm lens, so I would recommend the 100-400 as the best lens for getting started in bird photography. If you’re like me, you’ll keep this lens for a long time and get lots of use out of it for bird photography, travel, and dayhiking. If and when you graduate to a longer prime lens for bird photography, you’ll still want the 100-400mm around to use as your backup lens, or for mounting on your spare body for bird flight shots and close action shots.