Summary: This lens lets ton of light in through the shutter, which makes it very fast lens, very well built, very good pictures. You can read about it in professional reviews, and you'll see it gets pretty good ratings from everyone. It's quite wide and heavy so I needed a new camera bag for this, but if I have to take just one lens on my typical vacation, this would be it.
Summary: I needed a wider angle, all-purpose type of lens for everyday shooting. I almost got a "nifty 50," but at the last minute chose this one. I may still get a nice 50mm, but this lens is gorgeous. I recently used it on a wedding shoot and the pictures turned out beautifully. The only bad pictures were the photographer's fault (me), not the lens. It's slightly wider angle makes for getting some great shots.
Summary: I have been shooting for quite a while on kit lenses and mid grade lenses. And though I had heard a number of negative reviews about old Sigma lenses, I've found this generation to be of great quality. The price to quality ratio is way higher than I expected, and this is about as close to professional grade as you can get without going to the Canon L-Series lenses. If you're looking for most bang for the buck in upgrading, this is it.
Summary: I love the sigma "pro" lenses. Very comparable to the canon L lenses in most respects. I bought his over the 2.5x higher priced canon lens. I also own the sigma 85mm 1.4 lens which is also great. Their service and support are great too unlike tamron who couldn't bother to call me back after several messages, emails and calls to their support staff when I had a problem with one of their lenses.