Why do Canon charge so much for one piece of plastic? This compatible hood is half the price and appears to do the job just as well. It fits on a bayonet type quarter a turn mounting, and there's a reassuring click at the end of its travel so I'm sure in normal use it wont come off.
I bought it for my 17-85 IS USM lens and it parks on the lens in reverse for storage if required, if the hood does fit other lenses I assume it would do the same.
The initial reason for buying was was only really for 'show' to be honest, but having the hood I've realised that it's benefits are two fold; a small amount of protection for the lens glass from the weather but more importantly physical protection. Having a battery grip causes the body/lens to tip forward when put down; the hood acts as a cushion or shock absorber, especially if the largest part of the petals are on the top and bottom. The plastic is fairly flexible so I think it adds protection to my investment.
A worthwhile addition to the camera bag. Leave it on all the time and this ten pound circle of plastic should make you sleep easy at night. Recommended.