re: Diet sodas -- how bad are they for you?

They are most certainly not good for you. I'd say once a day is too much. The ingredients are unnatural chemicals that will most likely cause inflammation. Unless you are exactly where you want to be health and fitness-wise, I would seriously cut back. And yes, artificial sweeteners do cause insulin release; this has been studied.

This healthline article is also helpful at explaining the negative effects of artificial sweeteners