I bought Sigma 10-20mm couple weeks ago and so far I love it. I know people have been saying that some copies are kind of soft at the right side, but I've been checking mine since I've got it and so far it seems like I got a really sharp copy. Just wanted to share for those who are looking for a UWA lens, this is a really good lens for its price.
Here's a sample shot I did.
ISO 100, 1/250, f/5.6, @20mm
And second Image is 100%, Shot in RAW, and nothing was done to the picture, uploaded as shot.