[sirdarksol]

Aspertame has not been shown to cause negative effects in most people...
However, I agree entirely with DocBrown. The stuff, at the very least, has the ability to break down into formaldehyde (wow, got the spelling right on the first try), and does so in at least a small percentage of the population. It's my belief that it does so in all, but most people's bodies can handle the small amounts.
Still, I wouldn't want to be consuming even a small amount of formaldehyde if I could avoid it.
In people with fibromyalgia, apertame is horrific. My wife spent about a year in utter agony because she was drinking diet soda in an attempt to lose weight (more on the irony of that in a moment). Fibromyalgia patients' bodies can't get rid of the formaldehyde quickly enough, and it builds in their muscles.

On to the irony. In addition to all this, there's likely little point to using sugar substitutes unless you're diabetic and need to watch your sugar intake. Several studies have reported that artificial sweeteners trigger the same body mechanism that causes you to store calories as fat. Other studies have shown that children who drink diet soda have the same increased chance of obesity as children who drink regular soda.
Your best bet is to just avoid foods that have high fructose corn syrup, and stay away from sweets as much as possible. When you do have something sweet, try to find something that's sweetened with cane sugar, raw sugar, or crystalline fructose, as these are slightly harder for the body to break down than HFCS.