|I've owned the Nikkor 12-24 f4DX, but sold it cause it often hot spots when shooting IR (important to me). I've had the Tokina 12-24 f4 for only a couple of weeks now, but I must say I'm quite impressed with it. Forget the money difference I think head to head it is as opticaly good as the Nikkor, and is better built. Sure it has a little more CA in real high contrast situations, but that can fixed in Adobe RAW.
I borrowed a Sigma 10-20 for a day from a friend before I bought the Tokina. Basically for me at least the extra few degrees of coverage wasn't worth the bump in speed. And although I didn't have a 77mm polarizer with me to test it, I've read that especially at 10mm it can be hard to filter.
However, according to all I've read on online, you can't go wrong any of the new wide zooms, although many think the Tamron is over priced compared to the others.