No soda is healthy, and even though the Pepsi Bottling Company lists 0 calories in the soda, there is a lot more to consider than just simply calories and fat. The most damaging part of a diet soda stems from the added replacements for sugars, which some consider to be cancerous.
The most damaging effect of diet soda actually stems from the very thing that makes them diet in the first place. By replacing the natural sweeteners, usually with high fructose corn syrup, this actually triggers higher insulin production in the body. The result is that the body goes into fat storage mode, which results in weight gain.