Like Amy said, it's complex to use and doesnt save money, but is a great tool to use to prepay. We have used the dinning plan twice. We booked way in advance and wanted to pay for as much as we could while we had the money. We are horrible at saving money. In the future we'll buy gift cards if we want to prepay.
They do not give change for the vouchers. So, say its $5 snack voucher and you spend $4. You either need to spend more and get over the $5 and pay cash for the difference or loose the $1. The vouchers are not good in downtown disney, and some of them are only good in the parks not at the hotels. They take FOREVER for the cm's to enter at the register. I've heard they are going to a scan /bar code system, but havent seen it yet.
If you do decide to go with the plan, I recomend you buy a couple days less then then length of your say. Say 3 day plan for 5 day trip.