I've been suckin' down diet soft drinks for years, but have also added in the much cheaper bottled water. This doesn't imply that *all* bottled water is cheap, but as PTB said, there's a sucker born every minute, and two born to take him.
I never was a coffee drinker. I used to drink almost nothing but caffeinated soft drinks (Coke/Dr. Pepper) since being a toddler. Gallons a day. When I got really ill, I had to stop cold turkey and switch to fruit punches. I had a headache that lasted for two months when I quit the caffeine, ugh.