Do a Google search for weight gain diet soda. There are a lot of articles saying that, while drinking diet soda may not directly cause weight gain, it can stimulate appetite, lead to increased belly fat, and increase the risk of metabolic syndrome.
The most popular soda is Coke, which comes in two no-calorie flavors: Diet Coke, and Coke Zero. Diet Coke uses asparatame exclusively. Coke Zero uses asparatame plus a newer invention called Ace-K. Most people prefer the taste of Coke Zero, because it's more like normal Coke. Other sodas' "diet" varieties typically follow Coke's naming and use just asparatame. I wonder if soda companies are not just rebranding, but also looking into reducing the asparatame taste...