Julia, I have always been a diet Coke drinker. I don't like the full sugar stuff at all. But the diet stuff, I found, was giving me a false sense of hunger. I would crack open a can right after I got to work, and then have my second can around 9 am. By then, I was starving. So most mornings now I do not have a diet Coke; instead, around 9 AM I make myself a big mug of hot tea and eat it with a bar of some kind that actually has some nutritional value. It gives me that little mid-morning break without getting me all juiced up on the diet Coke.
So for me, I would probably blame diet Coke for getting my stomach rumbling and thinking I was hungrier than I really was. In that way, it probably contributed to my weight gain. But I can't be sure about that. I mostly have noticed that my migraines are greatly reduced; for that reason alone, I'm happy to have cut way, way back.