I bought the Canon 10-22 and lived to *almost* regret it. I found it was sharp stopped down, but a bit soft wide open. It came with no lens hood. Once you add up the lens plus hood, it's over $700.
I bought the Tokina 12-24. It is sharper than the Canon wide open and equal to the Canon stopped down. Comes with a hood for $500. It is constant f/4 where the Canon is f/5.6 at the long end.
The 10-12 mm range of the Canon is still great to have, but for me the higher build quality, constant f/4 and increased sharpness wide open and cash in my pocket make the Tokina a better choice for me than the Canon.