I've been shooting with the 12-24mm f/4 DX for years, and I find it's "just about" wide enough for me most of the time, although I occasionally chafe a bit with it. I know that when I make the transition to FF, I can use my 17-35mm f/2.8 and get about the same WA, but I've been intensely curious about the Sigma 12-24mm.
As I expected, at 12mm on a FF DSLR, there's a pretty significant distortion effect, but then, that's pretty much fisheye territory anyway, so I shouldn't be surprised. Indeed, it's a fair comment that the distortion is so well controlled for that degree of wide-angle. It obviously does get that bit further than the Nikon 14-24mm, costs less, and for the most part, I'd be shooting it stopped down for DOF anyway.
However, I look over my 12-24mm DX lens shots, and I see that I don't shoot exclusively at 12mm, actually spanning the FL range pretty well in my work. While I occasionally chafe with not getting more WA, I obviously don't need just the widest end of the zoom.
Has anyone seen or taken photos with the Sigma 12-24mm at other FLs than 12mm ? I'd be curious to see just how it works.