I used to drink a lot of diet coke as a kid. Kept getting recurrant headaches though. Someone suggested it might be aspartame so I cut it out. The headaches stopped pretty quickly and I've never suffered from regular headaches again.
It really annoys me that it's in loads of non-diet stuff though. I have to keep an eye on labels and I think I have a really low tolerance to it.
There is no proof that the aspartame was causing the headaches, it could have been the caffeine or something else