Customer Rating: 
Summary: Shockingly good
Comment: Far exceeded all of my expectations. And my expectations are very high. I still can not believe the amount of detail and the contrast this lens delivers. I have 1.8 20mm Sigma lens, which I thought was pretty good, as long as I applied some sharpening in post-processing. But this thing blows it out of the water completely.
Customer Rating:
Summary: Excellent Lens,
Comment: This lens takes very clear images, its extremely fast and offers low-light image taking. I am using this lens exclusively right now because it offers so much flexibility between indoor and outdoor lighting.
Customer Rating:
Summary: Sigma hits the sweet spot!
Comment: What a fantastic lens! I've always used sharpest lenses (17-55 f/2.8, 50 f/1.4 etc). This lens is not only super sharp, but it's like the best L-lenses: it gives superb micro-contrast, bokeh, color and realism.
I use it on my 40D with a Kenko Pro1 UV filter. My favorite apertures are f/2.8 to f/5.6. But, f/1.4-2 are not bad either if there is not sufficient light.
The Sigma AF is not as reliable as Canon ones. My 30mm doesn't front- or back-focus consistently, but AF can miss focus at times, especially if you use it at f/1.4.
My favorite zoom is 17-55 f/2.8. I wouldn't leave home only with 50mm, 85mm primes, but 30mm is much more versatile than those primes and I don't hesitate to use bring lens as my only lens to an event or a trip.
30mm is quite appropriate for portraits, but if you want the ultimate portrait lens, I would recommend Canon 50mm f/1.4 (or f/1.2L).
Customer Rating:
Summary: Sharp lens, but inaccurate auto-focus
Comment: I've had this lens for about a year and a half now. The lens itself is small and easy to carry in a camera bag. I can confirm that it's also very sharp. When it's manually focused it can be incredibly sharp, but unfortunately the auto-focus is inconsistent and will often front-focus (i.e. the red auto-focus dot will light up on an object, but the picture ends up focusing on something slightly closer than the object supposedly in focus). I've done testing on a tripod and compared it against manually focusing the lens and I confirmed the issue does exist. I've read, however, that Sigma has issues with consistency and quality control sometimes, so it's entirely possible I just got a bunk lens and that others will be fine. Note that Sigma reverse-engineers the auto-focus systems from Canon/Nikon/etc. because those manufacturers don't provide data sheets on how to interface with the camera, so Sigma's lenses *are* more prone to error than the "native" lenses, and I'm certainly not the only one to have this error.
More about the lens: at f/1.4 the contrast is a little weak and it's not as sharp as if you stop it down a little more (to say f/2.8), but it's still better than most zoom lenses at any aperture. At f/2.8 or higher, everything is in perfect focus. This lens has the potential to be the perfect prime lens because it's the equivalent to a standard ~50mm lens after the multiplier of cropped sensor cameras like the 40D/30D/20D and XSi/XTi/XT, plus at f/1.4 it's great in low light. But with poor auto-focus, the lens becomes far less useful than it otherwise would be. Using manual focus it's perfect, though.
Customer Rating:
Summary: Great bargain lens
Comment: One of the best lenses you can buy for your Canon crop camera.(20/30/40D or any of the Rebels)Many people report problems with this lens but like Jim Hunt said in another review, I believe most of the focusing errors are operator errors. Prime lenses and especially prime lenses in the "normal" range have very few moving parts and are far less likely to have mechanical issues. That said, I love this lens. I have moved on to a full frame camera so I can no longer use it. For the money the Sigma 30mm is one of the best lenses you can buy. One reason I believe so many people report having problems with this lens is the price brings it into the affordable range for the amateur photographer or student photographer. These are the photographers that are least likely to understand Depth of Field, and the least likely to do proper testing and evaluations. Therefore I believe there are some errors sure, but they are not the fault of the equipment.
Summary: Shockingly good
Comment: Far exceeded all of my expectations. And my expectations are very high. I still can not believe the amount of detail and the contrast this lens delivers. I have 1.8 20mm Sigma lens, which I thought was pretty good, as long as I applied some sharpening in post-processing. But this thing blows it out of the water completely.
Customer Rating:
Summary: Excellent Lens,
Comment: This lens takes very clear images, its extremely fast and offers low-light image taking. I am using this lens exclusively right now because it offers so much flexibility between indoor and outdoor lighting.
Customer Rating:
Summary: Sigma hits the sweet spot!
Comment: What a fantastic lens! I've always used sharpest lenses (17-55 f/2.8, 50 f/1.4 etc). This lens is not only super sharp, but it's like the best L-lenses: it gives superb micro-contrast, bokeh, color and realism.
I use it on my 40D with a Kenko Pro1 UV filter. My favorite apertures are f/2.8 to f/5.6. But, f/1.4-2 are not bad either if there is not sufficient light.
The Sigma AF is not as reliable as Canon ones. My 30mm doesn't front- or back-focus consistently, but AF can miss focus at times, especially if you use it at f/1.4.
My favorite zoom is 17-55 f/2.8. I wouldn't leave home only with 50mm, 85mm primes, but 30mm is much more versatile than those primes and I don't hesitate to use bring lens as my only lens to an event or a trip.
30mm is quite appropriate for portraits, but if you want the ultimate portrait lens, I would recommend Canon 50mm f/1.4 (or f/1.2L).
Customer Rating:
Summary: Sharp lens, but inaccurate auto-focus
Comment: I've had this lens for about a year and a half now. The lens itself is small and easy to carry in a camera bag. I can confirm that it's also very sharp. When it's manually focused it can be incredibly sharp, but unfortunately the auto-focus is inconsistent and will often front-focus (i.e. the red auto-focus dot will light up on an object, but the picture ends up focusing on something slightly closer than the object supposedly in focus). I've done testing on a tripod and compared it against manually focusing the lens and I confirmed the issue does exist. I've read, however, that Sigma has issues with consistency and quality control sometimes, so it's entirely possible I just got a bunk lens and that others will be fine. Note that Sigma reverse-engineers the auto-focus systems from Canon/Nikon/etc. because those manufacturers don't provide data sheets on how to interface with the camera, so Sigma's lenses *are* more prone to error than the "native" lenses, and I'm certainly not the only one to have this error.
More about the lens: at f/1.4 the contrast is a little weak and it's not as sharp as if you stop it down a little more (to say f/2.8), but it's still better than most zoom lenses at any aperture. At f/2.8 or higher, everything is in perfect focus. This lens has the potential to be the perfect prime lens because it's the equivalent to a standard ~50mm lens after the multiplier of cropped sensor cameras like the 40D/30D/20D and XSi/XTi/XT, plus at f/1.4 it's great in low light. But with poor auto-focus, the lens becomes far less useful than it otherwise would be. Using manual focus it's perfect, though.
Customer Rating:
Summary: Great bargain lens
Comment: One of the best lenses you can buy for your Canon crop camera.(20/30/40D or any of the Rebels)Many people report problems with this lens but like Jim Hunt said in another review, I believe most of the focusing errors are operator errors. Prime lenses and especially prime lenses in the "normal" range have very few moving parts and are far less likely to have mechanical issues. That said, I love this lens. I have moved on to a full frame camera so I can no longer use it. For the money the Sigma 30mm is one of the best lenses you can buy. One reason I believe so many people report having problems with this lens is the price brings it into the affordable range for the amateur photographer or student photographer. These are the photographers that are least likely to understand Depth of Field, and the least likely to do proper testing and evaluations. Therefore I believe there are some errors sure, but they are not the fault of the equipment.