I bought the card. At first they would bring us a small cup where we had someone in our party who did not get the card and with a can of soda they got was more than twice as much. So we had to hound the waiters for soda more often b/c of the small cups. Also they weren't too happy getting us the "free card" sodas. No tip for them.
As I look bak it really is a toss up. Next time I'm paying as I go.