I bought this lens to replace the Nikon 14-24 with my D700. I also have the Nikon 24-70 (and 70-200 VR2) and do not often use a lens below 24. The 14-24 was therefore quite a waste.
However, I wanted something much cheaper that is not too different in terms of results. After extensive research (reading almost every review of every wide lens I could find) I bought the Sigma on amazon for a great price - £ 350 used. Everywhere else was around £ 500-550.
It arrived in its box and was as new. It is really good to use. I have been used to using the Nikon professional lenses and wondered how disappointed I was going to be.
In fact, I am thrilled. It is a great weight and size, definitely better than having to carry the 14-24. It covers a great range (12 is quite a lot more than 14), the AF is good and it is very sharp.
Some reviewers said that the Sigma was quite soft. I disagree. At least the one that I have is excellent. The colour is also very good and contrast is fine.
I did a blind test of my wife and kids to see which they preferred. Everyone chose the Sigma sample pictures. I can see why, it is great.
If you are looking for the very best, get the 14-24. There is nothing better. If not, consider the Sigma: you will save a fortune and the difference will be minor. A great lens.
PS my 14-24 is available for sale: great condition