Depends entirely on what/where you eat and what you would eat if you weren't on the plan.
Plan meals include desserts, for example, which you may or may not order if you were just paying for food.
For most people, I would say - and definitely for our usual group - it doesn't really save us anything (maybe 5%) but we like the prepay-no thinking thing. Of course, when it's "free", it saves us a little bit more sometimes, especially when we do character meals and such.