20 years ago I was classified as "obese". For a long time I had drank about a 12 pack a day of regular coke or mountain dew. I switched to drinking a 12 pack a day of diet mountain dew. That had no effect on my weight, but the change was worthwhile in eliminating the need of multiple new fillings every six months when I went to the dentist.
In the past ten years I've given up all sodas, among other changes in my diet, and have now got my weight down to the point that I'm just below the "overweight" category. Down about 50 pounds from where I was back when I was drinking the diet mountain dew.