Since I'm upgrading to a new full-frame D700 next week I decided to buy a Sigma 12-24mm f/4.5-5.6. This is the widest lens that has ever been made for a full-frame camera. Sigma's current batch of lenses have a great build quality, along with the HSM, and great pricing you so you usually can't beat 'em.
I already had the Tokina 12-14mm f/4 for use with my DX cameras so I decided to do some quick test shots with both lenses on the D300 for comparison.
Both lenses are built quite well, but the Sigma looks and feels more like a pro lens. Since I generally shoot these ultra-wides at the widest setting of 12mm I decided to only test the lenses at this setting (also to save a little time).
At the center of the frame at the widest aperture, f/4 for the Tokina and f/4.5 for the Sigma, the Tokina was sharp, surprisingly sharp. The Sigma was soft, much too soft.
The Sigma didn't show any improvements in sharpness until f/11 at which point it was almost as sharp as the Tokina at f/4. At f/16 and f/22 the Sigma was softening again due to diffraction. This was a disappointing performance to say the least.
The Tokina was sharp at f/4 and continued to get sharp up to f/11, at f/16 it started to soften from diffraction, but was still sharper than the Sigma at f/11. At f/22 the Tokina was pretty soft. The Tokina is amazingly sharp for an ultra-wide zoom at this focal length. Very impressive.
In the corners of the frame the Sigma performed even more poorly than I had expected after looking at the center sharpness. I was very surprised by this since the Sigma is a full-frame lens the corners should be much sharper due to the 1.5X crop factor.
At f/4 the corner of the Sigma image is completely devoid of any fine detail. It also shows quite a bit of CA. At f/5.6 it's a tiny bit better, but not by much. When set to f/8 the sharpness is a lot better. At f/11 it's still pretty good, but at f/16 and f/22 you see signs of softness from diffraction. The CA was pretty much under control by f/8. Again, a pretty poor performance, especially from a full-frame lens. If the corners are this soft on a DX camera, I can't even imagine how bad they will be on an FX camera. That being said, you do have to consider that this is the widest lens available for the FX format and you have to make some concessions for optical quality when trying to build an ultra-wide lens.
The Tokina was pretty soft at f/4 with some serious CA, by f/5.6 the sharpness was great and the CA was under control. From f/8 to f/16 the sharpness was acceptable and the CA was OK. At f/22 the corners were softened from diffraction. The Tokina was a pretty good performer in the corner sharpness, but not awesome.
The Tokina came out the clear winner in my book, something that came as a complete surprise to me. I really expected the full-frame Sigma to be much sharper especially in the corners. Of course both lenses suffered from some CA, but that is to be expected in lenses of this focal length. The Tokina is also about $200 cheaper than the Sigma so you definitely get more bang for your buck.
Although the Tokina is sharper, I'll probably keep the Sigma due to the fact that it will work on my D700. I wouldn't use this lens for critical work or for big enlargements, but the fact remains that this lens will give you a 122º angle of view on a FX camera, which is really wide. You just have to decide which is more important for your work, image quality or the ability to go super wide.
Hopefully, I'll get my hands on a Nikkor 14-24 f/2.8 soon and I'll add to the comparison.