Ok, as i have bought this lens i thought i would give my impressions of the first 6 hours.
This lenes build quality is pretty darn good, certainly a big step up from my 17-85 and 50 f/1.8, but probably not quite as good as the Canon version, which is meant to be bomb proof, but it is still very good. The 'EX' finish on the lens is very nice and i like it alot. In product photographs it may look quite rough but it is actually very smooth and has a lovely texture to it. I will also point out that the tripod collar is a brilliant deisgn.
The AF is very fast and would say it is as fast as my 17-85 (which is very quick) in real life situations, this is pretty impressive, although it is a bit noisier, which is to be expected as it is moving a substantial amount of glass in comparison.
Unfortunately my copy front focuses, (badly @ 70mm and just a bit out @ 200mm), so i will give a full impression of it's optical qualities after it has paid a visit to the Sigma service Centre
For all those thinking of buying one in the Canon mount, please be wary that i so far have not heard of a single copy coming straight from the manufacturer, which has AF correctedly.