the rates are different for kids under 13 than for teens and adults. I want to say it is $34 for a 7 day cruise for kids and $48 for teens. I just purchased this for clients last summer but can't remember for sure. Let's put it this way, even though bringing your own soda on can be a pain, unless you drink an awful lot I wouldn't but the card. Of course, I am a good one to talk, I don't drink soda..
I bought the card when we cruised on the "Sun" in February. It was convient to just go bring your cup up for a refill, but it's fountain soda out of a "gun" so sometimes the mix was good and sometimes it was too syrupie and sometimes it had too much water. I think I will just bring some on board with me and then purchase my diet coke/diet pepsi at the ports. I actually bought Dt. Coke and Coke Zero in Belize, Cozumel and the Bahamas and didn't find the taste too different then what I was used to here in the states. St. Thomas is our first port so I should be able to buy a supply there.
Like was previously mentioned this posting, it all depends on how much you drink and if you want to bring some on board with you and if you would feel comfortable buying some in port.
You are right about the mixture from the soda fountain "guns" being variable depending on the location of the bar. I think that the only soda that was consistent at all of the venues was Ginger Ale.
As far as bringing your own soda back on board, NCL does not allow its passengers to bring drinks back onto the ship. You may be able to sneak a couple of cans in your bag but it is hardly enough to last for any lenght of time. One of our traveling companions tried to bring some tonic water on board (two bottles) and they showed up on the x-ray machine and were not allowed to go to the room.