It seems that diet drinks with artificial sweeteners can successfully fool the taste buds but not the brain. For example, after drinking an original Coke with syrup the brain will detect sugar in the bloodstream and inform the body that it's not so hungry, thereby curbing the appetite.
Artificial sweeteners don't do this and the consumer will be just as hungry afterwards. Bearing in mind that when energy is required, the first things to be burnt up are the sugars, if you indulge in something hot and greasy (steady now!) after your diet drink, will you not put on more weight than if you've had the original syrup sweetend drink because fat is only used after carbohydrates to provide energy?