This is a very interesting lens, because it doesn't have the best reviews online and has been recently discontinued. It was actually the world's first SLR lens with the Image Stabilizer feature built in.  For a first generation product I think this lens does exceptionally well, and it's a lens I very much enjoy using.

The biggest selling features of this lens is easily its long zoom range, the IS capability, and the price. This lens is great for shooting sports outdoors, wildlife and even macro photos.  The 300mm reach is excellent for getting close up shots of action and wildlife without having to get too close.  One of my favorite shots (The flying Monarch Butterfly) was done with this lens.  I could not have gotten this shot with any other lens I use.  The 300mm reach allowed me to get the close-up while the Image Stabilizer meant there is very little blur in the photo due to my hand shaking while chasing around that butterfly.

This lens while it has the USM title is not actually a Full Canon USM lens, which means it isn't quite is quick but it also means there is no Full Time Manual focusing. You will have to flip the switch on the lens from AF to MF before using the manual focus ring.  Other than that this lens is very solid in construction, and although it is heavy, relative to lenses of this zoom range it is fairly light.

Canon actually has a few different versions of this lens.  Starting from the cheapest in price, there is one with new USM features at all.  A lens which is the same as this except with out the IS, and this one with the IS.  Canon also has a new version out which has a wider range from 70-300, improved IS but still no true USM.

While most people find this lens not sharp enough or fast enough, I do like it. I was skeptical at first but quickly grew to love the images I captured with it. While it's not as sharp as my other lenses, it captures images I otherwise wouldn't have been able to capture.