The quality of the filters in your TV set have a big influence on the quality of S-Video and even Composite picture quality.
If your set has a stellar 3D adaptive comb filter, you can get an excellent picture.
I have a Sony PVM 20m2mdu, and even my NES on composite with the right settings looks brilliantly clear and sharp, but the color is a little muddled compared to RGB, and whites have more clearly defined edges.
My Panasonic Viera Plasma is another story.
The filter in that is pretty standard, and you can see the artifacts and blurry edges where the colors bleed into each other, even over S-Video.
It's a mess.