Recently purchased this lens after seeing the great reports of "macro" version of this lens. The reports were correct, this is a gem of a lens.
I comparison shopped between this and the similarly priced Tamron 28-75. What I can say is either I got the best copy out there, or the Sigma negative reviewers were really unlucky in the early 2003 copies of the lens, or (I suspect), Sigma has really focused on making this a whole new lens since it's re-release as a macro edition. This lens is outstanding!
I obsessed over a CF-card full of comparison shots between this, the Tamron 28-75 Di, the Sigma 24-60, the Sigma 28-70, and the Canon 24-70L. (Originally was going to get the Tamron after it's great review on photo.net this month).
What I can say is my results showed this Sigma as matching or even beating the 24-70L image-wise, and ever so slightly beating the Tamron 28-75 at full aperture (this will *not* be noticeable unless you really go out of your way though, the Tamron is outstanding in it's own right, just showed a tiny bit of fringing on some backlight letters), and it really wipes the floor with it's 24-60 and 28-70 cousins.
After testing it extensively, I'm very happy I bought it, the lens is so sharp and clean on my test, it comes almost indistinguishably close to my primes in this range, (the Canon 50mm f/1.8 and the stellar Sigma 50mm f/2.8) in terms of color and sharpness.
I settled on it in the end over the Tamron because I liked the coverage down to 24mm since I'm shooting on a 300D. If I have anything to improve, it would be to make it smaller (it can be a beast to lug around, but so is the Canon 24-70), lighter, and to put in the HSM motor they've got in their 12-24. The HSM is the only thing missing for this to fully match the overpriced, somewhat overhyped Canon L.
This 24-70 now is a different beast with latest "Macro" version.