I recall reading a study some time ago that showed that Diet Cola drinkers actually had higher average weight and slower rates of weight loss than others. But I couldn't find the reference. Finally through my ultra nerdy Obesity+ McMaster obesity reporting service I received the citation to the article.
A study published in Diabetes Care this April followed the food intake questionnaires of patients over many years. The results were interesting. Daily or more consumption of diet soda increased risk of metabolic syndrome (syndrome associated with obesity and cardiovascular disease) 36% and the risk of type II diabetes by 67%. It seemed to be linked to higher waist circumference and high blood sugars.
How does that make sense? They do not elucidate possible reasons in the study because it's impossible to establish causality in such a study. However, I imagine it has something to do with the human tendency to overcompensate. You see, if you replace that 150 calories cola with a diet cola, that means you can have that 150 calorie snack you've been wanting all day. Problem is, we're really lousy at guessing how many calories things have. Furthermore, having diet soda often gives you a false sense of security, like buying indulgences to account for your sin.
I know you've seen it before: the overweight customer rolling up to the McDonald's counter and getting the double Big Mac meal super-sized with....Diet Coke. You're better off just throwing out the carbonated drinks altogether and replacing it with good old water.