Eh, I try to drink as little diet soda as possible, toxic or not. The problem is that my family never drinks regular Pepsi, and at times I get thirsty for something that isn't water or milk. But that's what my Mountain Dew stash is for, I guess. They say Mountain Dew rots your teeth and causes sterility, but, oddly enough, I'm not concerned about that right now.
As far as aspartame goes, I never put it in coffee, or anything else for that matter. I have eaten packets full of synthesized sugar at restaurants out of idiocy and boredom before. They're not half bad.