Photo.net have an interesting head-to-head comparison of the pro-level Canon EF 300/4L USM lens and the consumer Canon EF 75-300/4-5.6 IS USM lens.

“Despite lots of negative comments on the Canon 75-300 lenses, I think many of them may stem from users who simply don’t know how to use a 300mm lens. This could be because the 75-300 lenses are seen as “starter” lenses for beginners, and so tend to be bought and used by less experienced photographers. They may be shooting handheld at shutter speeds too low for sharp images, they may be shooting moving subjects or they may not be optimizing their images with a good image editor. I don’t know. I do know that the EF 75-300/4-5.6 IS USM is a decent lens. It’s not as good as a 300/4L, it’s not as sharp and it focuses significantly slower, but when used properly and with the help of a little digital magic, it’s a lens capable of yielding pretty sharp images. This especially appplies when shooting digital if, when at 300mm, you stop down a stop, optimize the image in PhotoShop and don’t try to make huge prints.”

Photo.net - Canon EF 300/4L USM vs. EF 75-300/4-5.6 IS USM Lens Review