Siman23, I use a 12-24mm nikon alongside a 17-50mm Tamron. The Nikon is great; well built, good optics and very wide field of view, so perfect for everything from editorial to landscapes. I have no problems with it whatsoever. Probably the one niggle about it is the ability to flare in strong sunlight on occasions, but that's due to a big-rounded front element and I expect that's the case with the competition too. The lens hood usually sorts that out (or a well-placed palm). Other than that i love it.
That said, from what i've heard and read, the Tokina is just as good and a lot cheaper; I went for the Nikon because it was at a knockdown (£450) price. If I'd not had that option it would have been the Tokina simply because I've played with the Siggy 10-20mm and I just don't rate it.
I fit mine with a UV filter or circular polariser and there's no vignetting but if you stack Colin P filters on top of the CP then you start to see darkened corners. It's not a problem if you recompose a step or two backwards and crop in later (or use the zoom).
AF on the Nikon is very quick an accurate. Minimum focussing distance is good too - think it's about 25cm. I use mine with a D2X and it performs brilliantly for magazine work.
Here's a sample pic to show the difference between 17mm (top) and 12mm (bottom). Both at f/4 with IO 640 and 1/60th. Shot in manual and handheld. Think the Tamron probably underexposes by about 1/3rd stop, there or thereabouts but at least it gives you an idea of what the 12mm offers over the 17mm.
Excuse my desk - it's a mess
