The original Sigma 12-24 was something of an odd lens. It was the first of it's type and had nothing that could compare or compete for many years. I bought mine when it was first released to compliment my then new 300D! Back then, getting a decent wide lens on a 1.6x crop was very difficult. Most of Canon's offerings were very expensive and not very wide at all. This was the only option back then.
When I got my 5D...wow...this lens really was WIDE! But it suffered from really mushy corners and dark dark vignetting in most apertures. I had to us it at either f11 or f16...anything else and the corners where terrible. But it was so well corrected for straight lines...there hasn't been anything this wide and that well corrected since. There was no straight line distortion..period.
The MkII was a massive upgrade in terms of sharpness and edge sharpness. The Vignetting was well controlled but the one key factor was that it wasn't so well corrected any more. In fact it had quite strong barrel distortion at 12mm, which was a pity.
This new lens looks very promising. It treads a nice line between sharpness, distortion, vignetting and corner sharpness. For an occasional use...poor mans 11-24L I think this will scratch a certain itch.
It'll be interesting to see how it compares to the Irix and Laowa prime offerings.
I also find that Simga launch prices are quite laughable. They tend to settle down after a year or so.