There is a good comparison i was looking at the other day of the Canon vs Sigma thing at FM
's if you havn't seen it yet.
There was also some shots from there in the Canon foru, showing the 10-22 in detail at different F stops and focal lengths.
Just like Canon Sigma has Qc issue;s with their lenses and if you find a good 12-24 it is super value from some shots i have seen both on paper and on the web.
The only hard part is finding a good one!!
The 10-22 looked real average wide open at 10-14mm but looked OK
when stopped down to around F8.
Probably good for landscapes but i'm not sure how good it will be on the street in low light.