We have done both.
I find that the Disney ones are great, but a little expensive. However, they are fairly durable and if they ever rip or any issue comes up, you just bring them back to any shop at the Parks and they will give you a replacement.
That being said, I got $1 ones for our upcoming trip. We have the Mickey ones from past trips, but this trip I just wanted something cheap and easy and I wanted to be able to pick my kids out from the masses more easily. I found ponchos in the Target $1 spot and got DD and I bright pink ones and bright yellow ones for DH and DS. Plus, I got an extra one to put over the stroller.