We've been using this Sigma AF 18-200 lens for a couple of months now. Recently we compared the Sigma with Canon's 28-135mm image stabilized lens on a Canon 30D. We shot side by side photographs at 28mm & 135 mm both, set to ISO 100 with camera in Program mode. Our lenses were set to autofocus with image stabilization on.
The good news which I really didn't expect was to see both lenses produce images that were nearly identical-I thought for sure Canon's optics would far surpass Sigma's. Color saturation, light exposure through the lens, and detail were indistinguishable. The autofocus worked equally well on both. Both weighed about the same and were about the same size. The Sigma has a 72mm diameter aperture which I liked (so does the Canon). Of course the Sigma is 18-200, and at this time Canon does not have a competing option in the same digital SLR lens class with image stabilization.
The only aspect of the Sigma that differed significantly from Canon's lens was the noise of Sigma's autofocus motor. Sigma produces noticeable motor noise, though it wasn't overly loud. Still if you were shooting in a spot where silence was critical such as a ceremony, it may become an issue but probably not. In comparison, the Canon autofocus motor is silent and fast every time.
Sigma places a locking mechanism on the lens barrel to eliminate any possibility of lens creep (lens extending when pointed down). I doubt this would be a problem with the Sigma anyway. The zoom ring is a little stiff, but not too stiff. The manual focus ring is designed to be used with AF set to off to avoid manipulating the motor when focusing.
The image stabilization works well allowing you to shoot lower light photos without a tripod. I was quite satisfied with Sigma's newest effort here.
I'd say this Sigma makes a great choice for amateur photographers like us looking for a single lens option on Canon digital SLR cameras. The only dissuading factor 'might' be the motor noise if you hope to match Canon's silent motor system. Until Canon produces the same class of lens at a competitive price, Sigma has this round sown up.
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UPDATE: "LENS CREEP" September 18, 2007
We did just notice the Sigma lens starting creep for the first time (extend while pointed down). So apparently while the zoom mechanism starts off stiff enough to hold the lens in position, over time it will loosen up enough to move on its own with gravity assisting. Still love the lens though.
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