[These comments are made on the basis of looking at dozens of images, shot over several days, not just the few examples on the previous page]
The Tokina 12-24mm f4.0 is a good lens. It's sharp in the center and pretty good at the edges when stopped down a little. Corners are a somewhat soft wide open at 12mm, but sharpen up nicely by f5.6. Similarly corners at 24mm wide open are a little soft, but stopping down to f5.6 sharpens them a little and stopping down to f8 sharpens them a little more. Center sharpness is good at all apertures and focal lengths. Chromatic aberration is well controlled, flare resistance is good and distortion, though noticeable at 12mm, is low. Vignetting is slight and not likely to cause problems and is essentially eliminated by stopping down a stop.
Probably the biggest factor in deciding between this lens and one of the other options (Canon 10-22, Sigma 10-20, Tamron 11-18) is whether you want the extra coverage that 10mm can give you over 12mm. If you are happy with 12mm, then the Tokina lens at an average price of around $500 may well be a better buy than the Canon EF-S 10-22 at around $690. The Canon lens has a silent USM motor and full time manual focus, as well as a wider angle of coverage, but it's almost $200 more (plus you have to buy the lens hood separately!).
In terms of optical quality the Canon may be a little sharper in the corners wide open at the wide end and the Tokina may be a little sharper in the corners wide open at the long end, but overall the optical quality of the two lenses is pretty similar. If forced to say which is the overall sharper lens, on the basis of pixel peeping at resolution test chart images I'd have to give the edge to the Canon 10-22 - but based on real world shots it would be pretty hard to tell. The Canon does show a little less distortion at the wide end of the range, though distortion in the Tokina is also pretty well controlled. The Canon 10-22 is also about 6.5 oz lighter so if weight is critical, that could be a factor worth consideration.
Overall I can say that I liked the Tokina 12-24mm f4 and I'd have no hesitation in recommending it if the focal length range meets your needs.
Where to buy:
You can help support this site by purchasing through Amazon.com. I use them myself and I've been very happy with their prices and their service.
Adorama also carries all these lenses and is a well established photo retailer based in New York.
Acknowledgement: I'd like to thank Lee Cullivan of Ziplens.com for making the Tokina 12-24 available for review. Lee runs an interesting service providing loans of various lenses in Nikon and Canon mounts. Renting a lens before you buy it can give you a chance to try it for yourself, or if you only need a lens for a special occasion, renting it is a lot cheaper than buying it!