EX glass is pretty good. Sometimes the focus or zoom ring is not where you thought it would be because of Nikkor lens usage, but the images are very sharp and clear.
Build quality is quite good. The finish feels like a very low pile velvet. If I could change anything, it might be that. Takes some getting used to. But it seems to hold up well and clean easily.
I have 5 EX lenses.
SIGMA EX PRO ZOOMS
. Sigma 24-70mm f/2.8 EX DG AF
. Sigma 120-300mm f/2.8 APO EX DG AF
. Sigma 300-800mm f/5.6 APO EX AF
SIGMA EX PRO TELECONVERTERS
. Sigma APO EX DG 1.4
. Sigma APO EX DG 2.0