For what it is (fast zoom of moderate range) and what it cost, this lens does not disappoint. Sharpness is acceptable at f2.8, very good throughout the frame by f8.
For wide angle landscapes focus is more consistent if you focus at 50mm, then pull back without refocusing. Use AF lock (custom function 4 set to 2 on the 20D). Focus consistency with this Sigma is no worse than my 50mm f1.8 Mk1.
The size and weight of the Sigma balances nicely with the Canon 20D, in my hands.