I wondered just how bad a lens this could be, given it's current overall rating by many reviewers found here. I'm glad I gave this lens a try anyways.
I took a look at the MTF design data at Sigma and compared it against Canon's MTF design data and quickly realized something. Sigma optimizes for high contrast around the center of the field, while allowing the edge performance to drop off pretty dramatically. Canon optimizes many of their prime optics for even performance across the field at the cost of absolute performance in the center at wide apertures. Take a look at the design data and you might see what I mean.
I purchased the lens for use in selective focus work. The need for something a little wider in terms of aperture and focal length came up after a recent shoot with a 28mm f/2.8 Canon (which I dearly love). I didn't mind the corners going soft as this is the kind of "look" I was after.
In the real world, my 24 Sigma f/1.8 offers nearly identical performance as my 50 Canon f/1.8 II (used on a 40D). In the center of the image the 24 Sigma is sharp and contrasty wide open and chromatic aberrations are well controlled. The edge performance wide open does degrade, and as the design information suggests, it does this in a linear predictable manner. By f/2.8 the 24 Sigma is truly outstanding across the field and matches (if not exceeds) the performance of my L-glass.
As a general purpose "walking around" prime optic on a crop sensor DSLR the 24 Sigma exceeds my expectations. As a selective focus widest possible aperture at a quite reasonable price, the 24 Sigma is worth every penny I paid for it.