Super zooms (like 18-200) tend to loose out in image quality to convenience.Personally, I feel there is big difference. Because I was using Kit lens 18-55mm non-IS and now I using Sigma 18-200 IS. And the color with the canon Kit lens is much more richer than Sigma lens.
Selective colour! wow... the Sigma has a good taste for colour!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:
Fresh analogy. I thought we already beat the apples and oranges to pulp already. haha.To the TS: That's like saying Americans are all much more plump than Asians when you have Michael Phelps over there and horribly overweight kids here.
It doesn't make sense to generalise like that. Each company will have their strengths and weaknesses in different lenses.