Just a heads up: I just got rid of my Sigma 18-125 3.5-5.6 cause it had a bad case of front focusing on my Rebel XT. I used it on our last trip to DW and after shooting 600+ pics, about 60% of them came out sharp. You'll see on Fred Miranda that some users are reporting the same issue. In hindsight, I should've researched it a bit more before buying, but we were time crunched, as we got the camera & lens 1 week before going on vacation. I was wondering why some pics were blurry, but thought it was my "noob"ness with DSLR's. Not so. I found out that it was a widely reported issue way after B&H's return policy had expired. You can get around it by refocusing once or twice more if you're using AF (not good for action/candid shots) or manual focusing (I'm not that quick, nor are my eyes that good).
My advice: check the focusing on the lens immediately after receiving it, that way you can exchange it until you get a good copy. Other than that, I really liked the lens. The pics that came out sharp were very good looking, the 3.5 gave enough depth to the background for decent portraits, and obviously, the focal length is money.
I now have the Canon 17-85 f4 IS. Decent lens, IS allows me to handhold in low light/no flash situations pretty decently, but not a real eye-popper in general. F4 not a real plus when taking portrait style photos (getting the 50mm 1.8 to fill that gap). And expensive.