The math works out pretty well on this one. A can of soda has approximately 150 calories. If you consume one per day, 150 * 365 = 54750 extra calories per year. One pound of fat contains approximately 3500 calories, so 54750 / 3500 = 15.6 pounds.
According to the article, consumption of the high fructose corn syrup that one normally finds in soft drinks does not cause the body to produce insulin. Insulin helps the body convert food calories into energy, so the "stealth" calories in soft drinks go straight to the body's energy reserves: fat. And unless you're doing an exercise routine that is specifically designed to burn fat (ie. vigorous exercise on an empty stomach followed by a low-carb meal), those fat calories are going to want to stay right where they are.
I used to drink several cans of soda per day. I was young, my metabolism was fast. But then it slowed down, the extra calories caught up, and I started gaining weight. I got that under control by exercising more and eating/drinking less. I brought myself down to about one soda per week, and stayed at that level for many years. Recently (within the last couple of months), I have gone completely soda-free. I'm not really sure why I stopped drinking that can of Coke at work, but it might have been any of several factors: (1) soft drinks are $1 at work, not free like they have been for the past 15 years, (2) I'm not getting tired during the day so I don't feel I need caffeine, (3) I am consciously aware that not drinking soda is better for me.
On my desk at work, there is one unopened can of Coke. It's been there for a few months now. I know that if I want it, it's there for me ... but it's warm. I'd rather have a cold one. That'll cost me $1 from the vending machine. No thanks, I didn't really want it anyway.