Sigma 17-50mm f/2.8 EX DC OS HSM - £570
Autofocus is ultrasonic but not as fast or near-silent as the ring-type system on the Canon lens. The Sigma also lacks full-time manual focus override, and the focus ring rotates during autofocus, which is a bit of a letdown when it comes to handling.
Vignetting is only really noticeable when combining the widest-angle zoom setting with the maximum f/2.8 aperture but, unlike Canon lenses, this can't be tuned out in DPP.
At f/2.8, sharpness is very good at the centre of the frame but drops off near the edges. That's usually not a problem; you'll most often use this aperture setting when you want to blur the background anyway. At f/4 and smaller apertures, sharpness is excellent across the whole frame. Overall, a cracking lens for the money.