Well, as somebody who drank at least 6 LITERS a day (it's a techie thing; and screw cans, the deposits add up too fast when you drink that much :P) for the better part of the last 10 years, I'm a pretty firm believer that there's a big correlation between soda and weight gain (I for one would never touch a diet soda, so the calories racked up).
I gave up soda on cold turkey on the first of the year after getting a rude scale awakening at a physical in December, and replaced it with cranberry juices, limeade, OJ and the like. I have not altered my eating habits in a dietary meaning one iota--if anything, I think I'm eating a little more calorically, as this winter I've eaten a lot more pasta than usual as I try to master the intricacies of Italian cooking, and as such have eaten less of my usual types of food...and I've still dropped over 30 pounds since the 1st of the year. I've still got another 30 or so to go before I get where I want/should be, but yeah...the soda has definitely proven to be a primary culprit to how I had gotten over the last couple years as my metabolism started to slow (aging) and my activity levels dropped.