Advice on Sigma Lenses?

I'm looking at buying my first lens for my 7D. Of course I've been looking at Canon lenses, but someone said I should also consider 3rd-party lenses, too. Primarily I'm looking for a wide-angle lense, since my 28-135mm that came with the body kit is a bit beyond wide angle (with the non-full-frame sensor).

What are the pros/cons with 3rd-party lenses?

How do you folks feel about Sigma lenses?

What about this lens:
"Sigma 20mm F1.8 EX DG ASPHERICAL RF"

Advice?