I have to agree with Dave on this. For adults, the soda cards just don't pay out. Unless you drink nothing but soda and drink at least three a day. I tend to drink too much wine or other adults bevvies to have it work.
Even for the kiddos, they don't drink enough soda to pay for it either. They have milk at dinner and an occasional soda. Since juice and water and the milk is free, they come close to having enough to pay for it, but not close enough. They would have to have about 2 glasses a day to break even.
On NCL the Kid's Crew offered a $14 soda card which was more reasonable (about 1 soda a day to pay for itself) But even then we didn't buy it.
(They and Carnival still allow you to bring your own soda on board)
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013