I've got a fair copy of the original Sigma 12-24mm f/4.5-5.6. It's fine at one aperture...f/11, which kind of limits its usefulness. It gets rolled out once or twice a year when the 16-35 f/2.8II just ain't wide enough and a stitch-up is impractical or impossible. Apparently the 12-24mm f/4.5-5.6 DG HSM II was a solid improvement on the original and I'm prepared to believe an Art 12-24 would be orders of magnitude better than my unloved clunker. I want one already.
I'm taking this thread as a reminder to sell the old 12-24 when I do my next annual spring-clean sale.
Yes, that's exactly my experiance too. I bought this lens literally the week it came out, back in the Eos 300D days. It really was the only wide option back in those days. Everything from Canon was silly money (16-35L mkI) and wasn't that great. But when I popped this lens on my full frame film camera...wow! So when the 5DmkI was launched...it became my main wide lens...I cobbled a filter system (way before anything commercial was available). But for landscape work, f11 is where it's at....it's really a one aperture lens. But in that zone, there has been nothing for over 10 years to touch it. I sold my copy last year and I've been getting by with my TS-e 17L. But there have been times recently where it's singular abilities would have been useful. I looked at the mkII but it didn't quite have the same look and feel the the images that the mkI had. Even at 24mm the Mk1 had a fully corrected look which I haven't seen in any other 24mm lens yet.
I'd like the Canon 11-24L...but that's a LOT of money for a very niche lens. So if Sigma bring out an art or contemporary variant with new optics which combine the strengths of both the mkI and mkII...I might well be interested. But I'm not interested in combining the weaknesses of both lenses.