I have a Sigma 18-50 f/2.8, but I haven't had it long enough, or really have much digital experience to make a judgement yet. It seems to work well, and I like it so far.
In the current Popular Photography mag (or one of the major mags) that old guy with the fishing hat (is his name Peterson?) does a review of the Sigma 18-50 f/2.8 and a cheaper Sigma with a similar zoom (the f/4-f/5.6 model or something). I just skimmed the article, but in the performance chart the f/2.8 lens got "A+" at all apertures in print sizes up to 11"x14", where f/22 dropped to "A". I'm not sure what that really means, except that the old guy with the fishing hat liked it, and that's good enough for me.
I don't know if it was the same mag, but I saw a different article where they did a shootout between prime lenses and pro zooms with DSLRs. I would expect a prime lens to be somewhat better than a zoom, but with todays pro zooms that cost as much as used cars I didn't expect to see much difference. I was really surprised at how much better the prime lenses (and just the standard, affordable models) handled subtle detail.