We bought Meal Vouchers through Cresta at the time of booking on our last visit, I ordered a mixture of Lunch and Dinner vouchers. I had at least one meal paid for everyday in advance, the only problem we had with ours was that they could only be used in specific restaurants and as we went in low season ( October), it was a little difficult to plan ahead, as not all the restaurants were open every day. Some of the staff seemed a little confused by them, but they were always accepted and we paid the extra for anything not covered by the vouchers. They have a face value and rather than being restricted to ordering from the Menu Pluto or Menu Decouverte as specified on the voucher, the meals were calculated on the tills and the voucher totals added up, so for instance when we ate in Annette's Diner although all 4 of us ate, I was given one of the vouchers back and some change.
If you want to plan your budget in advance they're a really good idea.