I bought this telephoto lens primarily for bird photography. The price is reasonable compared to other Canon telephoto prime lenses. It is rated 9.8/10 by 44 users at FredMiranda.com.
The built-in image stabilization allows me to handhold the camera even using shutter speeds less than 1/100 sec. It has a nice pullout lens hood as well!
This lens is sharp even at f/4 and the color/contrast are excellent. You can also find this lens on eBay.