I never liked the idea of aspartame, because it's a "natural" substance, albeit one that is a neurotoxin. Most people can metabolize Phenylalinine in food (yes, it's in all our food) in the amounts that are normally occuring in food. The reason newborns get blood taken from their heel is to test for this ability. If you don't have it, you will become mentally retarded. Babies with this metabolic inability must be fed a special diet for several years to let their brain develop normally.
Depending on the amount of aspartame one consumes, and one's personal tolerance, a person can create this metabolic disorder in their own body. This can range from headaches and neuropathy (tingling in the extremities) to seizures and blindness.
This is why some people eat it with no problems and some people, like my son, get a headache from half a diet soda. Many years ago, when he got headaches from diet sodas, we called the FDA and found it that was a common side effect. The man at the FDA said, pretty firmly, "Don't let him drink it." Which we did not.
Because of the science outlined here, I stay away from it.