Summary: I like to shoot sports and nature. The lens is light enough to carry around all day. 300mm is okay but 400mm would be better. The lens isn't long enough for small birds in flight. The lens is very sharp from 70-200mm. However at 300mm wide open, the sharpness isn't there. The long end and wide open aperture for a zoom lens is where it's most critical. Using this type of lens I find myself being at the long end of the lens most of the time.
Summary: The lense produces good pictures but there is one very notable problem. When zoomed from 200mm to 300mm the autofocus is not at all accurate. This is serious enough that I will not use AF when zoomed in this range. I get very nice, sharp pictures even at 300mm but only with manual focus. I've run a test to verify the problem is with the lense and not my 40D camera. MF is tedious and takes time so you will miss some shots when zoomed out.
Pros: Durable, Easily Interchangeable, Good Optics
Conclusion: This lens was my first zoom telephoto purchase. Initially, I loved it. It has a fantastic auto-focus, the zoom range is great for wildlife, and it has a durable body. I have taken many excellent daylight photos with it, and although it gets more difficult at sunset, it can be done. The most striking problem with this lens and the reason I would not recommend it to anyone is the lack of sharpness at full zoom.
Summary: This is a decent lens, there isn't anything too special about it. It does have IS (Image Stabilization) which is supposed to allow 4 addition stops of shake-free shooting, but I often find many shots come out soft. It must be noted that this lens beyond 200mm is noticably soft. I find it very difficult to get a sharp shot from this lens.