If so, you should take a look at this study MSNBC put out:
Apparently people who said they drank two or more diet sodas a day experienced waist size increases that were six times greater than those of people who didn’t drink diet soda, according to researchers from the University of Texas Health Science Center at San Antonio.
“Data from this and other prospective studies suggest that the promotion of diet sodas and artificial sweeteners as healthy alternatives may be ill-advised,” said study researcher Helen P. Hazuda, professor and at the university’s school of medicine. “They may be free of calories, but not of consequences.”
A second study that found the sweetener aspartame raised blood sugar levels in diabetes-prone mice.
“These results suggest that heavy aspartame exposure might potentially directly contribute to increased blood glucose levels, and thus contribute to the associations observed between diet soda consumption and the risk of diabetes in humans,” said study researcher Gabriel Fernandes, professor of rheumatology and clinical immunology at the university. You can see the full article here:
It makes sense that if you drink a lot of soda, whether its diet or not, you are still going to gain weight.