Summary: Great lens; sharp and fast, like I want my women. Image stabilizer a must for hand holding on wildlife shoots and shooting moving models 2.8 allows for a faster shutter to insure sharper imagery at higher shutter speeds.
Conclusion: I've shot with a 30D for about 7 years and recently switched to a 7D. All my digital SLR experience has been with these APS-C cameras. I purchased the 300mm IS f4L as a way to get a little more magnification than the 70-200mm IS f4L that I already had. With the APS-C format this lens performs similar to a 500mm (480mm), so it works well for bird and nature photography.
Pros: Fast auto-focus, Superior build quality, image stabilization, size and weight, close-up capability, case tripod mount & built-in lens shade
Conclusion: I have used this lens for a year now and am generally happy with its performance. I moved to this lens from a EF 100 - 400mm IS because I had heard that prime lenses give better image quality. I think the autofocus is a bit slower and there is a very annoying "clunk" that is well documented with the IS of this lens. I shoot whitetailed deer and usually can get a couple of frames shot before they hear the lens and dart off.
Summary: I am still in the learning phase with this lens. So far so good. Very clear pictures...since it is a prime and not a zoom...and due to its length, I am trying to get used to running around for the perfect shot at ball games.
Pros: Durable, Easily Interchangeable, Nice Bokeh, Strong Construction
Summary: I purchased this lens for wildlife photography primarily. For the price, it's a very nice lens, quick to focus and having the wonderful IS. I also like the fact that the hood just slides down. I did feel that I needed extra reach though, so I purchased the 1.4x extender. Even with this knocking things back one f-stop, it still does well on moving subjects, and I am able to hand hold it fairly easily for short periods of time.
Pros: Consistent Output, Durable, Easily Interchangeable, Quick Focus, Strong Construction
Summary: I've been shooting for over 15 years and I have to say this lens is as good as they get. With my 40D and cooperative wildlife, I get professional results, the results that get published and sell. I've used this lens in cold weather and it has performed flawlessly. This lens is always on the camera and is ready to go, all the time.