I own two SIGMAs. One is my favourite lens. It's the MACRO 105mm / f=2.8. It has a verry good built quality and resolution is extremely good. There's no distorsion, CAs or other remarkable failors. AF is a little bit slow, since it's not HSM, but still good. AF range can be limited for non macro usage.
The other SIGMA I own just arived today. It's the 10-20mm /f=4-5.6.
Same good built quality but ultra fast HSM AF. Can't say much about optical qualities at the moment since I did not have time to test. But according to
http://www.photozone.de/Reviews/46-...-20mm-f4-56-dc-ex-hsm-lab-test-report--review
it seems to be the best wide angle available at the moment. Distorsion is verry low for a extreme wide angle.
And not to forget; both lenses are Made in Japan! All original Nikon equipment I have is made somewhere else (China, Malaysia, Thailand).
So far to say, the EX series is verry good. Taking the price tag into account they are extremely good. Might be different for the low end (non EX) series.