I have several Canon bodies, including the 1DMkIII, 5D and 20D with multiple L lens for serious shooting. I use an XTi and 300D for casual photos.
I decided to purchase this lens for my XTi and 300D. Currently using the Canon 17-85IS and the 70-200 f4 on the XTi. These lens plus a 50 f1.8 and 550EX flash make a larger, heavier bag than I sometimes wish to carry. This lens allows me to take the camera and external flash in a small bag. The 300D has the terrible kit lens and a 55-200 - equally as bad).
Certainly not L quality, but it will allow a wide angle shot and then close-up in quick succession without a lens change or two bodies. Something very handy for casual shooting.
When not used with the XTi, it will rest on my 300D which is my beach and Jeep camera, a vast improvement over the original kit lens setup.
I am very satisfied with the build quality and price. I had purchased a Canon 28-200 last year and took it back the same day, as it was terribly soft. Much better with the Sigma (however all my other lens are Canon)