The cheap ones are great to have "just in case," but not reallly to use more than once. I get the ones from Target's camping section - they're folded down even smaller than the ones from Dollar Tree, and you can easily put one in a pocket or purse. They're about $1.
I'd recommend buying a little more expensive poncho if you're going during a rainy time. I got some ponchos at Target for about $8, and they were great and easy to re-use. They're made of more substantial material than the cheapie ponchos -- they cheap ones are very "clingy" when wet, and stick to your body, especially your legs, while you're walking. I felt like I had to pick away the plastic from my legs just about every step - very irritating!
You can also buy the WDW ponchos - they're around $8 each and meant to be reused.