Philip Greenspun , Aug 30, 1997; 01:08 a.m.
People do keep asking questions like this. What are we to make of them? Are the laws of physics going to be repealed for a particular lens? Are the laws of magazine publishing going to be repealed so that a new heavily advertised product gets a bad review? Is there some doubt as to whether Sigma and Nikon are competent at making lenses that perform adequately given their price?
In general, Nikon lenses have better construction, better optics, faster apertures (in some cases), less distortion, and higher resale value than third party lenses. Are these things worth the extra money? Nobody can tell you. You haven't said whether you're an engineer who appreciates finely constructed things or whether you ever intend to sell your lens or whether you do architectural photography (where distortion can be destructive).
Even if there were someone here who'd dumped his Sigma 18-35 recently in favor of a Nikon 20-35/2.8 lens, I don't think he'd have much to say that you wouldn't find in Duh Magazine.