I have had some experience with the My Store Rewards as a buyer.

It is quite popular in the USA and I have bought from several sellers who use it. When I paid for my first order I was automatically entered into the scheme and a few weeks later I received a small refund into my paypal account from the seller.

Since then I have received regular newsletters from the sellers, making a feature of the Rewards program.  So far I have bought again from one seller where the discount was added to the invoice, and one who refunded to my paypal account.

I must say I buy from these sellers because they have the best prices for what I want....the Rewards program is just a bonus.  It does not influence my buying decision at all.