I first used this lens on a phototrek to Moab and Monument Valley last Oct. The lens was swapped back&forth from the N80 & D100 and yielded crisp,tack sharp images in daylight, sunrise and sunset shoots. I highly recommend this Sigma EX as well as several other EX's I own, most especially the 24-70 f2.8.
I used a Polarizer and also a ND gradient filter with this lens. Firstly, I screwed an 82mm 'UV' filter into the supplied protective hood and kept it mounted between shots, the front element is bulbous and always in harm's way.
With the D100, I noticed that an 82mm Polarizer when mounted on the protective hood did not show vignetting from 17-30mm. From 15-30 it was visible--if this did not effect the my composition I shot the image. When the vignetting at 15-17 was not acceptbable, I used a clear sheet of polarizing material (6in diam) purchased from Edmund Scientic held in directly in front of the lens, image quality loss was minimal and acceptable for my needs--I could not discern any artifacts in my prints.
When using the N80, the protective hood+UV was kept on between shots--when shooting the polarizer or ND gradient (Cokin XP size) was held in front f the lens against the petal hood. Since Oct Cokin has released an XP polarizer and lens mounts, I have not tried them but am quite certain the dedicated will find a way which is the main purpose of this post.