Summary: I can't write about the optics yet, as I just received this lens. However, I get the idea of caveat emptor when you buy used, but the casing is so worn it's difficult to see the numbers. Not only is there no lens hood included as promised, there isn't even a cap for the lens mount. Working the zoom sounds like a lifetime of sand inside. Not happy so far.
Summary: This lens is a good solid one. I got the best results using shutter priority but I use Manual 90% of the time. It has nice colors but you can ALMOST get similar results with a kit lens with the right lighting. It is up from three stars because of how it feels on the camera. It is smooth and solid when zooming and It has a nice rubber like coating which is head and shoulders above the plastic kit lens you get. Also 18-55 canon kits lens hood and filters work on this lens.
Into the Looking Glass (with apologies to Alice living in Wonderland)
InHisHand "planted-in-the-mid-west", Amazon
2 February 2012
Summary: Believe it or not, Alice, I connected this low-end Sigma to my 5D Mark II (full frame) for comparison against the Canon 24-105 f4L. My results: this Sigma lens compares favorably, with caveats. Remember that the Mark II platform is not quite as "sharp" as the cropped frame cameras (it has more pixels over a larger area but is not as densely packed per square millimeter on the sensor), making comparisons a bit skewed--in other words, all images on the Mark II are a bit...
Summary: This lens has surpassed my expectations. It is a very good light lens. The lens does not hunt to autofocus. It is fast enough on the telephoto end (f4 at 70 mm) that I use this lens as my general walkaround lens. This lens is good enough to use for professional work when you do not want to use the heavy pro zoom lens. It was one of the best ebay purchases of the past ten years for me.