The Sigma f2.8 EX 24-70 lens costs £299, about a third of the price of the Canon EF 24-70L.  (Some reviews say that on a crop camera there's nothing to separate them)

I find it very sharp wide open at 24mm (have a look at the first photograph at full size) - It's not as sharp at 70mm and f2.8 but fine once stopped down and fine at 50mm 

Click on any photo to see the full-size image taken directly from the camera together with EXIF data

REVIEW IMAGE 2707 Sigma 24 to 70 at 24mm f2.8 .jpg