You can't generalise like that. Some Sigma lenses (the high grade ones, like those Stratix has pointed out) do perform very well, and are really value-for-money alternatives to the originals on the market.
In fact, some Sigma lenses are the only options to go for when the original manufacturer does not produce anything for such purposes. An example would be the 12-24mm f/4.5-5.6 EX DG, the widest AF rectilinear lens currently on the market. Another example is the 20mm f/1.8 EX DG.
Here's a sample of the Sigma 10-20. No boost of saturation or sharpness. Selective colour work and contrast curve applied: