Boris:
The outline stroke effect is indeed somewhat nebulous, but it does have its uses. Even though cool stuff like applying a gradient to a stroke won't work (and you'd expect it would), some of the distortion tools DO work and you can get nifty effects using those.
Try this:
Begin by drawing 2 identical lines, and apply the Effect>Outline Stroke to one of them (in this case, the right one):
As you can see, not much difference there...
Now, go to Effect>Distort&Transform>Transform, and scale horizontally to 200%:
A little different...
Next, apply the Effect>D&T>Free Distort tool, give it a good yank:
Even though your stroke is still intact in its original form, the effects applied will modify it significantly. Knowing this, it's up to you now to fiddle with it and see exactly what you can do with it. Try applying an arrowhead (from Effects>Stylize), or a brush...
Make sure to experiment with the order of line items in the Appearance Palette, too, drag the effect up and down in the order of things, and add multiple strokes to see what happens.
HTH
Bert
Bert Philippus -