I had the original version of this card about 10 years ago. We got about a $4000 rebate on our GM car. We paid for a lot of the extras for our new house with it and the rebate grew quite rapidly. At the time the program was more generous than it is now.
IIRC, GM pays the dealer the rebate directly, so it doesn't effect the price of the car. We negotiated the best price we could and told them about the rebate card when they filled out the invoice.
We got rid of the card when we decided we didn't want to get GM cars any longer and started using mileage cards exclusively.
If you want to buy a GM car in the future this card may be an option for you, but as I said, the rebate program was originally far more generous than it is now. I would do the math and see if you could get better value with some other type of card first.
Mary