Here's some findings from an 8-year University of Texas study that I had read...
An excerpt from the study author:
"What didn't surprise us was that total soft drink use was linked to overweight and obesity," Fowler tells WebMD. "What was surprising was when we looked at people only drinking diet soft drinks, their risk of obesity was even higher."
"There was a 41 percent increase in risk of being overweight for every can or bottle of diet soft drink a person consumes each day," Fowler says.
One reason is that the artificial sweeteners in diet sodas create a negative hormonal response in the body that increases fat storing hormone production and increases cravings for more sweets and refined carbohydrates in the time period after consuming the diet drink.
Now, I haven't checked the credentials of the author of this article, one "Mike Geary, Certified Nutrition Specialist, Certified Personal Trainer" and his site certainly seems to have an agenda -- eg. sell his book/newsletter. But CBS ran an article citing the same University of Texas study, with them saying:
"What we saw was that the more diet sodas a person drinks, the more weight they were likely to gain," she says.
That finding was a big surprise, but it reflected what nutritionist Melainie Rogers saw in her work with obese patients in New York.
"When we would switch them on to diet soda off regular soda, we weren't seeing weight loss necessarily, and that was confusing to us," Rogers says.
So you could say while diet sodas don't have calories, they can increase calorie consumption by creating cravings through hormonal responses. In other words, they really are no better so you may as well drink what tastes good.
In any event, it's certainly an interesting subject to look into.
Links for the pages I pasted those quotes from: