Soda Cards are a good deal if one drinks Sodas. They are easily purchased after boarding or at anytime from one of the bars. Also it is said they are a little cheaper after the ship sails because they do not have to charge sales tax. The tax isn't that much so we always purchase one as soon as we board. Usually they will throw in a Logo Cup as a incentive on the first day.
Ooops, I just read that you have to purchase the Logo Cup extra. She said it was part of the deal.