Been shooting with this lens on my 30D for about a week now. Purchased it based on (and despite) reviews found on this forum. I had the opportunity to shoot with the lens and review the images prior to purchase. I love this lens. Sure, it's heavy and noisy. But it focuses fast, even in low light.
I have no problems that others have had with their lenses. This thing is crisp & clear, and colours are very good. There is very little quality loss at either end of the zoom, regardless of f-stop. What little loss there is I've been able to easily clean up in Photoshop. My largest print at the tiem of this writing is 24x36, and that's crip & clear.
This is rapidly becoming my walk-around lens, despite it's weight & noise. I find it's a joy to use, and love the images it produces.
I had planned on purchasing the Canon EF 28-135 F3.5-5.6 IS USM, but when I compared image qaulity, this won hands down (both were about the same shelf price).
Price was a factor in my purcahse, and I don't think that I could have gotten any better qaulity for my dollar. I now recommend this lens to anyone that brings up the subject of gear.
The negatives of this lens (heavy & noisy) are more than offset by quality and price. The funky MF/AF issue and the reversed zoom never bother me in the least. I find I can go back and forth between my Canon lenses and this Sigma without even thinking about it.
If you're considering this lens, find a store that will let you fire a few shots off on your camera, and see the results yourself.