Good medical journals like The Lancet (which is brit, I forget the name of the US one) have published findings on Nutra-Sweet including personality disorders, sleep disorders, etc. They also point out the formeldahyde factor, and there's plenty of info on that, it's a carcinogen (cancer-causing agent.) The test groups for nutra-sweet complained about it, but they decided the symptoms were negligible compared to the "gain" and ignored them.
Diet sodas save you about 3% of calories, that's all, and usually, that 3% goes away because of the mental tricks we play - like, I can eat this brownie because the diet soda cancels it out. So a 3% gain on calories is hardly worth drinking embalming fluid!