The soda package is expensive, as you stated. In order for that to pay off, you'd have to drink 3 sodas per day. If you think you'll be drinking that much, then it's worth it. Otherwise it's $2.00 or so for a can of soda, if I remember right. The soda package is unlimited, and it is fountain soda. They'll either give you a very small cup with that, or you can bring your souvenir mug, which you should get when you get your first soda onboard, or if you purchase the soda package on board.
If you do decide to get a soda at dinner, my understanding is the waiter will have to go to a bar to get it. I'm not sure if this holds true if you don't have the fountain soda package, as they may have cans of soda in the kitchen.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas