Yep, you'll have to pay for soft drinks, alchol. You can get coffee, tea, iced tea and fruit juice for free by the Windjammer Cafe. In the morning either in the main dining room or the buffet, they offer juices with breakfast with no charge.
You really have to do the math to see if the soda card is worth it to you. You have to factor in the length of the cruise and how much soda you will actually consume. Also, I drink coke at home (more than I probably should) but I found the free lemonade onboard tastier and I only had 1 coke the whole cruise. Unless you just live on coke, I doubt you end up saving that much in the end.
Also, remember if you get the coke card, you have to carry that "wonderful" take home coke cup around with you everywhere.
Carnival Holiday (1999 ?)
RCI Sovereign of the Seas 2007
RCI Enchantment of the Seas 2009