This was a replacement lens I bought later after I sold the Canon EF 17-40mm f4L. In many ways, the Sigma is quite a respectable lens. Very sharp in the center area, and distortion is well controlled if used carefully. It's also wider than the 17-40 and you see the different the 2mm makes. The built quality is quite good, being Sigma's EX series of lenses. Also, the price is about half of what you would expect to pay for the Canon 17-40mm f4L.

The bad:
* Far edges are blurry
* Sigma colour has never been my cup of tea
* Exposed bulging front element
* Can not take front mounted filters without vignetting
* Non Ultrasonic focus motor

Despite the shortcomings of this lens, I have become fond of its super wide angle of view. Any wider and you are in the fish eye lens territory. If used on the 1D series, the bad edges are masked by the crop factor of the sensor. On 1.6 crop bodies, you shouldn't see any bad edges at all.