From an IQ standpoint, the Canon10-22mm, Sigma 8-16mm, and Tokina 11-16mm are similar and all quite good. The Tokina is f/2.8 but has the narrowest range, the Sigma is widest but slowest, the Canon has the broadest range and is intermediate in terms of aperture. The Sigma cannot use screw-in filters, the other two can.
Before getting the 5DII and 16-35mm f/2.8L II, I had the Canon 10-22mm for my 7D - an excellent lens!
A couple of samples:
EOS 7D, EF-S 10-22mm f/3.5-4.5 USM @ 10mm, 0.6 s, f/14, ISO 100
EOS 7D, EF-S 10-22mm f/3.5-4.5 USM @ 18mm, 1/100 s, f/11, ISO 100