I agree Sigma quality is ok but I wouldnt consider it very good glass, a little soft and not as sharp as the native glass from the various camera co's.
One thing they have going for them is price, way cheaper than Canon, Nikon or Olympus glass. Some people find the quality acceptable but that depends what acceptable is.
I bought a Sigma DC F4-5.6 55-200mm for my E1 for $199 bought was not happy with the quality at all. Contrast and sharpness was not there at all.
I ended up selling it and purchasing an Olympus Zuiko Digital 2.8-3.5 50-200mm ED and never looked back. When buying good glass there's no way to get around paying cheap. When it comes to lenses you get what you pay for.
Here is an example of a comparison of the two lenses.