If I drink a diet soda, within 10 minutes I will have about a 8-9 headache, complete with nausea and sometimes vomiting, and fever and chills alternating at about 120-second intervals. This will last for several hours, until I can finally get the pain under control enough to sleep for 7-8 hours.
Also have a friend who is affected almost exactly the same way by the stuff. Nasty stuff.