It's funny but I was actually reading several reviews for the Tokina, Tamron, Nikon and Sigma yesterday. By all accounts the Nikon (12-24mm) was the superior lens but the Tokina (12-24mm) scored a very close second, in fact when you take into account it is half the price of the Nikon it suddenly hits the top spot.
I know many on here and elsewhere are happy with the Sigma though. I was going to ask for opinions on the Tokina 12-24mm as I would like to add a wider lens to my collection. The reason I didn't was because I already have the excellent Nikon 17-55mm and so I would be carrying around a very heavy piece of glass (the Tokina is the heaviest of the bunch) for the 12-16mm range that I would like.
But now you have come up with this, it's perfect! I'm sure that for Ken to say it is better than the Nikon lens it must be good. I know he comes out with some sweeping statements, but even if it isn't "better" than the Nikon I'm sure that it will be as good. I shall certainly be waiting for this lens.
Why wait? Well it's a faster lens than the Sigma and I love constant apperture lenses (the Sigma isn't). Many won't mind about the constant f2.8 as they will say that they stop down to f11 or more anyway when taking landscapes and that's fine for them. But remember that the apperture isn't just about the recorded image, the larger apperture will give a brighter image in the viewfinder and help with focusing and composition. If you are considering using the lens indoors as well I think it is a plus.
It is bound to have a pro feel to it (be very well made. Take a look at the weight and comments about the Tokina 12-24, I see this is at the same level in their range). This might be important depending on how you use your lenses. Again, many are happy with the Sigma but I am not sure about the longevity and build quality of the Sigma lens. I'd trust the Nikon or Tokina to take some bumps, rain and snow. Again, this may not be an important factor.
For me it covers the range that I feel I'm "missing" at times, without overlapping my current range. Plus, as I've said, I love constant appertures and all of my lenses at the moment are f2.8.
But...$600-$700 is likely to translate into £600-£700 over here
In which case the Nikon comes back into the equation (although I'd still go for the Tokina because of the range it covers), which then brings the Sigma well and truely back in to the equation because it can be had for over half the price.
If you can find an importer from Japan for this lens at say the price of teh Sigma, for me it is a no-brainer, get the Tokina. And if you do, order two as I'll have one!