I don't normally post reviews like this, but I thought I'd do it for this lens as I also own a Tamron 11-18 for comparison. Truthfully I've never been convinced by the Tamron, especially with it's dreadfully cheap plastic build quality for it's price. I eventually bit the bullet and bought the Sigma (there's not much other choice in this zoom range) and I'm quite pleased I did. Both lenses produce sharp images however the Sigma has far, far superior edge sharpness and distortion control. The Tamron appears to have the edge colour wise with slightly warmer and more saturated results, however it must be said the Sigma produces more natural, neutral images. Build quality wise - I don't know how Tamron dare charge so much for the 11-18 - the body is totally plastic, the focus and zoom rings have no damping and feel loose, and the lens feels plain flimsy. The Sigma on the other hand is well made, feels solid and weighty, and balances nicely on a D200. It's a simple design and there's no auto/manual switch on the Nikon version (no great loss since Nikon bodies have this functionality built in) and I guess this is how they keep the price down. The ultra wide range makes this a fun lens to use and I'm using mine as a walk around lens.
I've read much about Sigma QC however this is my second Sigma lens (I also have the 150mm APO macro - an outrageously good lens) and I've so far had no problems. Overall I'd say if you want value for money and a great lens to boot, you shouldn't go far wrong with the Sigma.