It is certainly better than regular soda in terms of letting you lose weight. However, there are so many drawbacks to diet soda for health reasons that we strongly suggest that you ween yourself off of this type of drink with the exception of special occasions.
The carbonation has an effect on your body that prevents calcium uptake into your receptors. Calcium and vitamin D are important for weight loss because they play a vital role in some of the mechanisms involved in the metabolism of stored fat.
Diet sodas also usually contain sodium and other metals that act on the same receptors as sodium thereby causing water retention.
Finally, people tend to drink far more diet beverage than regular beverage because the sweet taste tells your body that a carbohydrate is entering the system and when no carbohydrate enters the system, the body requests it by sending a sweet craving to your palette. This creates a cycle whereby your sweet craving is never satisfied.