I've had the Sigma 10-20mm for a few months now. Some of the images are incredibly sharp but sometimes, the left side is really bad. It's not easy to pinpoint the problem since it is intermittent. It will rarely show at 10mm or when the image is oriented vertically! But from 12mm to 20mm, I would say 1 image out of 3 has the left side out of focus.
Here an image here that demonstrate the problem.
Please, tell me what you think!
Sigma 10-20mm at 12mm, f/8, 1/400sec, ISO 200, on tripod.
Full image (no crop, resize 33%):
Cropped image (100%): looking at the streets name, you can see the exact point where the lens begins to show focus problem: