I bring a couple of cheap ponchos (from Target's camping section, for about $1 each) and also a nicer poncho, that's meant to be reused.
After several October trips where it rained pretty often, I got tired of the cheapo ponchos because they're made of a really flimsy material that tends to stick to your legs when wet. I got so tired of having to peel the cheap ponchos off of my legs every other step. Even the official ponchos you buy at WDW are made of a flimsier material than before, and they tend to stick to legs, too. Hate that!
So basically I keep a cheap poncho in my bag as a "just in case" if it doesn't seem like it will be raining much. But if the forecast calls for lots of rain on a particular day, I carry my nicer poncho -- it's made of sturdier plastic and tends to stand away from your body more (no sticking to legs!). It cost about $10 - I think I bought it at Target or Walmart.