I bought this lens for my Nikon D70 as a replacement for the 18-70mm variable aperture kit lens (which I didn't like very much). For a lens in its price range I think it performs well. If I had the budget, I would get the Nikon 17-55 f/2.8 DX instead of this Sigma, but I don't think it's useful to compare these lenses because they have a 3x price differential.
Sharpness: With medium to bright light, the Sigma produces images that are breathtakingly sharp in the center. I've taken several images inside my apartment. We have a couple oriental rugs and the level of detail the Sigma captures is truly amazing; you can see the silk patterns even at 18mm from several feet away. BUT -- we also have several wall hangings with nice textures, and they turn to mush if they're close to the edges of the picture.
Focus speed and sound: Noisy, but not any noisier than my Nikkor 85mm f/1.8D. A bit slow in low light though; you'll sometimes find the Sigma hunting around, especially if you're moving from a focal point far away to one close by.
Vignetting: Only slight. I take mostly environmental portraits and street shots with this lens, so vignetting is not really an issue. I am not the kind of guy to stop down the camera and point it at a white wall to check vignetting.
Build: Heck, I like plastic. It feels well constructed. I like the amount of resistance in the zoom ring. I do not like the cheesy speckling effect though.
Quirks: In medium to low light, I get blue channel values that are substantially lower than they should be. I can't reproduce this every time, but I am sure it is the lens and not the camera or operator error. This can be corrected in Photoshop, but it is a nuisance.
Overall impression at 18mm: Sharp as a tack -- substantially sharper than the kit lens -- in the center of the image, especially if you stop down a bit. Mushy at the edges, but not so much that it bothers me.
Overall impression at 50mm: Good images, not as good as my Nikkor 50mm f/1.8D, but still very nice.
I would recommend this lens as a replacement to the kit lens if you want it for walking around, environmental portraits, or street photography. If you need an architectural lens that's sharp from edge to edge, you might want to look elsewhere.