There are many threads out there on Club 91 - this should really be appended to put all the info in the same place.

We looked at this option last year before our Cancun trip was hurricaned away (before we arrived). I think a key factor is what you plan to do. If you are going to be at the pool all day every day and don't mind going upstairs for meals, drinks, etc. then maybe it would provide some value as food is expensive. Also a better deal if you book a cheapo room as I think you get a nice upgrade.

But if you are planning to do day excursions to Cozumel, Playa, Isla, etc. and to see the ruins, then perhaps the money is better spent elsewhere as you won't be saving much on meals. And clearly the program is a better value for families as you are essentially getting the kids in for free.