There is agreement in medical community that excess sugar consumption is directly linked with obesity and many other diseases. American Dietary Guidelines state that the biggest contribution of added sugars in our diets come from sugar sweetened sodas, fruit drinks and energy drinks.
Diet sodas are made with artificial sweeteners having almost zero calories. These are just chemical drinks with no nutritional value. Apparently you get the best of both worlds with diet sodas: you get to enjoy your favorite soda and you also do not drink any calories with it.
But it is not as simple as stated. Therefore, we need to examine two questions: Are diet sodas safe and whether they are a good strategy for weight loss?
Are Diet Sodas Safe for Humans?
Though there is no conclusive statement but research has shown that increased use of diet soda was linked with higher risks for metabolic syndrome and diabetes.  Another medical study has found a positive link between diet sodas and higher risk of stroke, heart attacks and blood vessels diseases. 
The research about which ingredients of diet sodas contribute to these increased risks making diet sodas unsafe is quite limited. In fact other studies have also been conducted which show no direct link between diet soda consumption and heart and diabetes disease.
However, you should keep in mind that diet soda is definitely not a food. It is a mixture of chemicals. For example, it contains phosphoric acid which affects tooth enamel and bone density.
The caramel color used in sodas have been linked with many different age associated diseases. And above all, the safety of artificial sweeteners has remained debatable and a few artificial sweeteners have also been taken off the market because of safety concerns.
Do Diet Sodas Help With Weight Loss?
On the fact of it, they should because you are restricting your calorie intake by shifting to diet sodas. But increasing amount of medical evidence suggest that diet sodas in fact cause weight gain.
Research has shown that diet sodas alter processing of sweet taste in the brain.  The brain’s reward processing system is altered in people who use diet sodas on regular basis.
Other research studies have shown that a consistent and regular use of diet sodas increase sugar cravings because artificial sweeteners used in diet sodas may be sometimes 700 times sweeter than sucrose found in normal sugar.
Both these factors perhaps cause weight gain because human body gets used to sweeter calories and craves for it. Many research studies have established that diet soda intake is positively related with increased waist circumference and weight gain. 
Should You Drink Diet Soda?
Going by the above medical evidence, it is recommended that you limit your diet soda intake to a maximum of one drink per day, ideally one drink per week. Moderal is the key. Try to use water as your primary drink to stay hydrated.