I did my homework on this lens, and I haven't been disappointed. I use it for architecture and tripod photography. I wanted a super-wide that would work on a full-frame camera. I considered the Canon, which I could have purchased. But, reports of average sharpness and distortion of the Canon L super-wides (previous or current models) persuaded me against those lenses. (Why pay a high price for mediocre quality?) I tried a couple of the Sigma 12-24mm's, and neither was acceptable. After seeing tests of the Sigma 15-30 on the old Rob Galbraith site, I purchased one new from Cameta.Com.
Now that I've used it for a few months, I like the lens. I regard it as a sharp lens. Not like my 24-70 L, but sharp. I knew in advance that it distorts. On my example, it pincushions above about 24mm and barrels somewhat below that focal length. I obtain acceptable corrections (for me) using the Photoshop Lens Distortion filters. (If I photograph above 24mm, I use a different lens.) I haven't noticed too much vignetting, even at 15mm. It's nice being able to photograph a little wider than I need. Both corrections for distortion and perspective consume edges of the image.
Did I get lucky with my example? It's hard to say. I think the production consistency is better than that for the 12-24mm. For whatever the reason, I got a nice lens for a reasonable price.