I recently bought the Sigma EX 120-300mm f/2.8 HSM after a lot of angst and web-searching. Glad I did, great lens, I love it. Fast, sharp, contrasty - all that stuff. I think it'd surprise you if you've been using a non-EX non-HSM Sigma. I have recently used a Canon L 70-200mm f/2.8 IS USM and I believe that the Sigma EX compares well to it on build and performance values, although both lenses handle differently due to their repsective sizes.
Anyway, it's something to consider ...