"Based on its sheer specs the Sigma 24-70/2.8 HSM EX is a very attractive lens. It promises professional grade results at a more attractive price point. It terms of mechanical build quality it is certainly up there with the best in its class with the exception of weather sealing maybe. The AF speed it a tad slower compared to Canon's USM but it's still comfortably fast. However, the key aspect is, of course, the optical performance.
At its optimal settings it is certainly a very sharp lens across the most of the image frame. The weakest focal length is at wide end. The center as well as the border quality is very high here but the extreme corners are soft from f/2.8 till f/8 and you need stop down to f/11 to get the best from the lens. The sweet spot is in the middle range especially when stopping down a little.. The corners are softer again at 70mm @ f/2.8 but decent beyond. Field curvature, a weakness of both pro grade Canon L standard zoom lenses, is not an issue at all. The vignetting and distortion characteristics are typical for a full format standard lens. Lateral CAs are very well controlled. The quality of the bokeh is quite disappointing but it's rarely really good in this class anyway. Bokeh fringing is very well controlled though.
So some aspects are really good whereas others may be less impressive but how does it compared to the Canon EF 24-70mm f/2.8 USM L then ? The Sigma may have a slightly worse bokeh but it does not suffer from a very pronounced field curvature problem like the Canon so its field behavior is more predictable."
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