The lens arrived this morning and I've taken a few field (non-scientific) test shots of the view outside my window. Here are my initial observations.
Centre at 24mm and f5.6 not quite as sharp as my 24-105mm at 24mm / f5.6. At f8 and f11, there really isn't much to choose between the two. The 24-105mm frames slightly tighter so either it doesn't quite reach down to 24mm or the Sigma doesn't quite reach up to 24mm.
Borders and corners very good at all apertures - at least as good as the 24-105mm. This is a pleasant surprise.
Resolution appears to be consistent from 12mm through to 24mm.
I've so far failed to induce any CA. It's a sunny day here so I tried sun reflections off shiny paintwork. No discernible CA. Another pleasant surprise.
I thought to begin with that the lens was back-focusing but it isn't. Being as wide-angle as it is, the difference between 2 metres and infinity on the focus ring is just a few millimetres of rotation so the distance window displays infinity when focusing on anything more than a few metres away. It just goes to show how deep the depth of field is on a lens like this.
Build quality excellent. Smooth, dry action to the zoom and focus rings (very similar firmness and feel to the 24-105mm).
I'll post some photos when I've had a chance to do a proper outdoor shoot.
Incidentally, I bought it from Warehouse Express here in the UK but I can't find anywhere else in any country that is advertising it yet.