I mostly drink diet to not gain weight and to feel full between meals. Plus I'm a very thirsty person (yes, I do drink water as well). Occasionally I have a regular pop if it's a special occasion or I'm hungover or something.
I switched from regular to diet about 7 years ago and hardly ever touch regular soda these days.
I'm trying to cut soda out for the most part (minus the occasional once a month treat or so) though so it's mostly water and coffee for now.