The Tokina 12-24 is an excellent lens. I just sold my copy (version II, with better coating) to transition my kit to full-frame ready. Another option others haven't mentioned is Sigma 12-24mm HSM (it's also compatible with full-frame, but that's another story), amazing IQ.
I've got to jump on the Tokina 12-24 bandwagon as well. Great lens. My budget didn't stretch to the 10-22 when i got mine, and I've never regretted it once.
There used to be some amazing "show us your ultra-wide angle shots" threads around here.... probably due for a "Show us your APS-C Ultra Wide" threads to show off some of of the Sigma, Tokina and Canon gems for 1.6x.
I have a Tokina 17mm f/3.5 prime that is a excellent buy for the price. Cost me $150. I liked it better than my 17-40mm L so I sold it. They are a bit hard to find, but bargains are out there.
I've got the Tokina 12-24/4 and I love it. I previously shot with a Canon 10-22 and I think the build quality is better on the Tokina and image quality is a toss up. 10 is definitely wider than 12, but I'm not sure if it's worth the price premium. I've been very happy with my 12-24.
For those who are worried about f4 vs f2.8 -- what kind of shooting do you do? You might need 2.8 if you mostly shoot concert stuff, but remember that a fast aperture is less important on a wide lens. You won't be getting a thin DoF anyway, and at 12mm you only need 1/24th shutter speed (using the 1/2*focal length rule).
