I bought the Sigma 17-70mm f/2.8-f/4.5 about six months ago when I was planning on buying a spare Nikon body as a back-up. I had a similar size Nikkor lens, but I wanted to keep that in my main camera bag. I bought the Sigma because it was an f/2.8 at the wide end. I payed £128 second-hand from eBay and didn't expect much. Six months on and I'm using this lens more than any other. The 17-70mm is one of the sharpest lens's I have used and it's not even one of Sigma,s high-end EX range (I have a new Sigma 10-20mm EX now too). If my gear got stolen tomorrow, this is the first lens I would buy again. The only downside is that it's not a fixed f/2.8 aperture.
I had a shoot last week for an album cover by Italian guitarist Will Barbero and the 17-70mm was the lens I used for 90% of the job (Nikkor 50mm f/1.8 made up the other 10%). The photo of Will bellow was taken by the Sigma and has been reduced from a 9.35mb raw file to a 43.3kb 640x425 blogable image. Taken at 40mm, f/5.6, 60th sec, 200 ISO on a Nikon D300 on manual.