Its hardly ancedotal or placebo... it took me 3 years of experimenting to narrow down what was causing the problems - its only since christmas that I've been 100% sure it comes and goes with my consumption of aspartame.
It took you 3 years to experiment that one substance causes it? The question would be, what did you take with the aspartme, and once you'd crossed off everything else you thought it might be, and concluded there was one option left, well, at that point as others have mentioned, the placebo effect can be impossible to ignore.
Either way, just because you react to aspartme, doesn't mean its bad for you. Some people are allergic to lactose, while the rest of the world is stronger using products with it in. SOme people are allergic to certain proteins, obviously this is not the case for most people and doesn't mean the specific proteins are bad.
It REALLY gets on my goat when one person, or even a decent sized group of people(that compared to the population is still indescribably small) have a problem with something and insist on telling everyone how it will kill them too.
Some people are unable to metabolise fat well and have to eat a carb heavy diet, most people will be much healthier on a lower carb diet, your brain is made of fat, a group of people not doing great with fat doesn't mean fat is bad for you.
Your logic that aspartme must be awful for everyone, because you don't get on with it, is simply flawed. We're all different, all of us, and its very likely that for almost any substance out there in the universe, there will be a few dozen people allergic to it while its fine for everyone else.
Even worse than that is the people who run around saying things are bad "because its a chemical".
We've even had that in this thread, aspartme's worse for you because its a chemical, and what is sugar exactly? People get all up in arms about manufactured chemicals, rather than natural chemicals or substances, which is mostly utter madness. Especially as most artificial things, well lots and lots of them, are often artificially reproduceable chemicals of substances found from natural sources.
Its unbelievable the amount of people who will happily take anything branded as natural, while shunning anything man made insisting it to be deadly.
Anyway, corn syrup or other really crap quality sugars they dump in the non diet versions are normally horribly bad quality cards, incredibly refined and are indeed bad for you. Crap highly refined carbs do more damage to the body than most small quantitys of chemicals. The body can filter out chemicals very well, its built to metabolise carbs and will only metabolise highly refined ones very quickly, the reaction of your body to high levels of sugar is far more dramatic, and long term, not healthy at all.
But they don't taste the same, more power to people who want to drink whatever the hell they like. Everything in moderation frankly, diet coke tastes better than normal coke, which leaves a "my teeth are melting" feel in my mouth long after I've stopped drinking, the nasty feeling they've reintroduced with Coke Zero, I assume by adding some chemical that acts to strip you tooth enamel.