I would suggest you google the card and you will find a wealth of info but from personal experience here's what I know:
We purchased our card last April through Sharon, Queen of SXM along with our cell phone rental, for a cost of $20 for the card. It was delivered to us along with our phone and rental car by Lesley Bruce and IMO, it was more than worth the money.
The card is good for a specified number of weeks (four I think?) from the time you actually receive it and is good for various offers from the companies involved. You can go to their website and it is all spelled out for you.
I believe each card is good for up to four patrons and I know there's a second card available which is geared more to residents or lont-time vacationers although I don't know the cost of that one. I would probably buy the visitors card and try it out and if I found it beneficial I'd get the second one for the rest of my trip.
Some of the offers include a percentage off on your dinner checks at various places while others were for glasses of wine or champagne coctails; there are offers for beach chair rentals and actual discounts for spa services and shopping options.
We used our card twice when we went to dinner at Moulin Fou at Porto Cupecoy and once for dinner at Maximos at the same location. I think there is also a match bet at one of the casinos and discount offers on some island activities.
Only you can decide if the card would be worthwhile to you or not but for $20. we felt it was a good option for our family on that trip. The champagne coctails and glasses of wine more than covered the cost of the card so IMO, everything else was icing on the cake.