My favorite mixed drink when I was an undergraduate was the Cuba Libre or as it's more commonly known, a rum and Coke, and being a purist, I never ever considered substituting a Diet Coke for the Coke.
For those who do use the low calorie option however, there may be an unintended consequence. A recent study shows that "mixing alcohol with a diet soft drink resulted in elevated BrACs [breath alcohol concentrations], as compared with the same amount of alcohol mixed with a sugar-sweetened beverage;" that is, one gets drunker. What's worse, the study shows that drinkers could not notice any difference in their level of intoxication.
In response to this study, the American Beverage Association countered that the "consuming calories - from any food or beverage - along with alcohol slows down its impact." The association represents Coca-Cola, Dr. Pepper, Nehi, Pepsi Cola, and Seven-Up.
Drink up, but drink wisely.