Got my hands on a Sigma 12-24EX today although just for a brief moment.
Have to say i am surprised with the results and it gives some nice opportunities, for sure when you are working inside a 747 fuel tank.
Also a former business class cabin looks different then it used to be.
It all stays quite sharp and the biggest benefit over my own 15-30 is that it doesn't flare.
An amazing piece of glass i would say and it is the first thing on my wish list.