We used the plan last April.
At the end of our trip we had several snack credits left over. Before jumping on the bus to MCO, I picked a few items in the food court, (water, soda, chips, cookies...) to snack on during our flight.
We had snack credits left over, because the dining plan doesn't cover the type of snacks we wanted during our trip.
We carry a water bottle around the parks, so we didn't need to use the credits for water.
A few times we wanted soft serve ice cream, but couldn't use the plan for that.
We couldn't use the plan for candy apples, caramel apples, fudge, funnel cakes, school bread, dole whips...
The plan was fine when we wanted popcorn, chips or a Mickey ice cream bar. However, we like to try the other WDW snacks as well.
We're using the dining plan again this summer. I'm sure we'll have several snack credits left at the end of our trip. I'm planning to use them in the food court before heading to the airport.