The soda card is a great deal though, my daughter got one and she used it by the pool, and during dinner. The dining room we were in allowed her to use it there as well. When we were by the pool, all I had to do was show her card, and the pool server walking around just took the order and I told them that she was waiting in line at the slide or in the pool and by the time she got back so was her soda and she didn't have to wait. She didn't have to be present when we were at the pool for her to get a drink. It was great! That was on the Destiny!
September 2008, Carnival Senstation, Bahamas
August 2009, Carnival Destiny, Jamaica/Cayman Islands
Whether a soda card is a good deal is subjective. Given the price, on a 7-night cruise an adult has to drink the equivalent of 25 cans of soda to break even. We calculated it a while back. It is a better deal for minors.
I don't bring personal soft drinks aboard so I can't say for sure, but the official Carnival policy says "a small quantity". That is subject to interpretation and actually could vary by port. You can read their policy HERE
A couple of 12-packs can look smaller than a full case. Especially when you have a couple of people in your group. Just food for thought.
They are not overly strict about this. The two twelve pack idea that Dave mentioned is a good idea. I usually take what is equivalent to a case of spring water, and some bottles of soda without any problems.