Simply put, this is the widest zoom lens you can get that'll still work on a full-frame/35mm body.
My top reason for getting this WA zoom was to have one that would work on both my Canon Digital Rebel (now upgraded to a 20D) and my Canon Elan 7E. This lens works wonderfully on both bodies.
Drawbacks, however, include: on a full-frame camera, it does vignette considerably; it's a fairly slow lens; it's a touch on the heavy side; the price I paid was a touch high.
Overall, I like the lens, and probably won't trade it. It's -great- outdoors where you've got plenty of light, but rather tough to use, for example, in a dark club for band photography.