Hi Folks,
I found a Tokina 12-24 on Ebay for $400, So I just couldn't resist giving it a try. I've wanted one of these lenses for a while, because they project an image circle suitable for APS-C cameras, however, they ARE compatible with other cameras, unlike Canon's EF-S mount.
So I can use it on my 1DmkII or on Sarah's 20D!
On the 1DmkII, the usable zoom range is 15-24 which is not too shabby. At 15mm the lens vignettes a bit at F4, but is good by F5.6 and great by F8:
F4 @ 15mm:

F8 @ 15mm:

And just for fun, here's a photo at F8 @ 12mm!!

Pretty cool, eh?
Mini-review: This lens is almost exactly the same as the $1,000 Nikkor 12-24DX, however, MSRP is $500. There are a few differences, as kindly noted by MikeO and repeated here. The zoom ring is at the BACK of the lens where it should be :-) The lens is the "AF-S" or "USM" type, so no screw drive = good fast focusing. The AF-MF is switched by pulling the focus ring back toward you (Tokina trademark). The lens focuses very close.
Most importantly, the build quality for this lens is SUPERB. Tokina lenses are built of higher quality materials than the Nikkor. The optics also seem to be superb based on my few test shots.
I would ABSOLUTELY recommend this lens over the Sigma 12-24 for APS-C users and for 1.25x crop shooters. It is smaller, does not have a protruding front element, and is almost as wide. I haven't tried Sigma's 10-20 yet, so I can't compare the two.
Lastly, here's a shot of the cruizer taken from 10" from the car:

Cheers
James