For near one year i use the Sigma 10-20 EX with my Nikon DSLRs.
Around 10 mm i retrieve the framings i loved with my wide angle lenses when i was shooting 24x36 mm films.
Some reviews told this lens was suffering of a strong shading but it's really not an issue on mine. I never noticed this although printing big frames.
Sharpness is good, the lens feels robust and the HSM focusing is very nice.
In my mind the only default is that some landscapes sometimes show a strong distorsion in their upper right angle.