I had the Sigma 70-200/2.8 HSM non-OS version. When comparing w/ my FA*80-200, the built quality is just miles apart. Focusing w/ the Sigma is very quiet and pretty fast, but it hunts a lot when using under low light, sometimes just gives up, while the FA*80-200 can manage to lock focus almost 100% times. I bought the Sigma to see if it could replace the heavier and older FA*80-200, but ended up I still keep the FA*80-200 and sold off the Sigma.
Optically I think FA*80-200 is slightly sharper too, at least my copy is.
I have similar experiences w/ the Sigma 24-70/2.8 HSM under low light too, but it's not hunting as badly as the 70-200 HSM. Optically I like the Sigma 24-70 HSM better 'cause it's very sharp even wide open, and seldom misfocus even under low light (assuming it's not hunting). That's why I'm keeping it. If Pentax comes out w/ a full frame dSLR, it'd be my most used lens