I was a little sceptical about buying one of these after a bad experience with a Sigma 28-70mm f2.8 EX some years ago. However, I seem to have a good one. It's not perfect - note some of the negative comments above. However, taking the lens as a package, it delivers outstanding value for money. It also has a three year guarantee, unlike Nikon who only offer one year here in the UK.
As with all ultra wides, they have to be used appropriately. By pointing the camera up or down from horizontal you will get converging or diverging verticals. This is normal and not a lens fault. Also, parts of the image at the edges will also appear more 'spread out'. Again this is a normal optical effect with wide-angle lenses and not a feature of this particular lens.
At a hundred pounds less than the Tokina 12-24mm, four hundred pounds less than the 12-24mm Nikkor and an extra couple of mm at the wide end, it seems like an excellent deal. Time will tell though if it the build is up to the mark, but I tend to treat my gear carefully.