Pros: Consistent Output, Durable, Strong Construction, Sharp Focus, Rugged
I choose this lens as a standard zoom for my 40D because it covers all focal lengths I use for indoor shooting (from group photos and interiors to close-up portraits) while providing fastest aperture available for DSLR zooms, which is absolutely a must have for shallower depth-of-field portraits ans for low-light photography. I was considering another similar lenses: Tamron 18-50/2.8 and Canon 17-55/2.8 IS USM, but the Tamron usually collects a bit lower ratings on Web than the Sigma and was not available in shops at the moment; the Canon got IS, USM, great reviews and superb sample images but it looks a bit overpriced (2.5 times higher than the third-party lenses - if I wanted to spend that much I'd rather upgrade to a full frame camera and L lenses); its main advantage is IMO Image Stabilization, but when I'm photographing people and get blurred images it's usually because of their movement, not because of camera shake. As for non-USM focusing, the lens is quite fast and not too loud (although focusing sound is clearly audible). So I decided to go for Sigma and so far I'm absolutely not disappointed. I prefer to never use flash and I shoot a lot of portratis so I almost always use the lens at f/2.8 and it works good enough on that aperture, giving pixel-per-pixel sharpness and with very minor CA at corners at widest angle. IQ advantage over Canon EF-S 18-55 is overwhelming, especially at shorter focal lengths, even at f/2.8 vs. any aperture of 18-55: better sharpness, contrast, way lower distortion, and etc. Can't directly compare IQ to another zoom lenses of this range as I'm mostly into prime lenses and only used a few other zooms of different focal range, but I'm sure it won't lose to any other zoom of this price range. Another advantage of the lens is 1:3 macro capability, which is more than enough for pets, flowers and so on.