I've always had mixed views on Sigma. On the one hand they do make some very good lenses, often for a lot less than the big names, and they do make some lenses that you probably wouldn't otherwise find anywhere near the same price range (for example a 30mm f/1.4). On the other hand their quality control doesn't seem to be great, and there are many stories of lenses not autofocusing properly. Also, when I looked at their film SLRs, I was very impressed indeed by what functions they put in the cameras - like mirror lock-up - which again would not be found on any of the big names' cameras at the same price... but again the build quality did not overly impress me.
One issue I have with Sigma is that they use their own proprietary lens mount. That may sound fairly silly; all the big names have their own lens mount too. But my point is, with Sigma you are limited to Sigma lenses, whereas with Canon, Nikon, Minolta/Sony or Pentax you have not only their own lenses but also Sigma's, Tamron's and Tokina's. This may or may not be a problem for you.
As for their digital sensors... I honestly don't know if they're any good. I have never used one. Hopefully someone else knows more about that.
