Rather than deal with anecdote, lies, or just plain bad information. Let's look at evidence. I don't regularly use aspartame since I'm not into sweetening things. However I never hesitate to use it if I want to. The key is, if you don't like it, if it makes you sick or gives you a headache, don't use it. I'd advise the same thing for any food or beverage as well.
Aspartame has been claimed to produce brain tumors. This claim is a misinterpretation of testing done on every product. New products undergo many tests before they are allowed onto the market. One of those tests is referred to as mortality testing. If you are an animal rights activist, stop reading. This test is performed on a few hundred animals, usually rats. The test is simple, you force the animal to ingest large amounts of the product until they die. If they don´t die, you give them larger doses. The purpose of the test is to determine at what level death will occur, and then after autopsies, the cause of death is determined. This test is done on every food, and always results in the death of the test subjects. In the case of Aspartame, the leading cause of death was starvation. Basically so much aspartame was given that the intestines could not absorb other nutrients necessary for life. The second leading cause of death were brain tumors. This test is the root of all the rumors regarding Aspartame and brain tumors. The fact is that no brain tumors in humans has every been attributed to Aspartame. The amount of aspartame ingested by the rats is nearly impossible for humans to ingest even if they were eating the raw product.
The second myth regarding Aspartame involves methanol and embalming fluid. In a laboratory, Aspartame can be converted to many of the same substances used in embalming fluid, however, the other chemicals needed to achieve this process are not found in the body. The other part of this is the rumor that Aspartame can turn into methanol. This is correct, but so can every fruit and vegetable and most other things we eat. Our bodies are very capable of dealing with the methanol that is produced when we eat food.
A more detailed STUDY can be found here-