I used to be addicted to Monster. I got to a point where my body had to have at least one a day to function normally. If iO didn't have it I would get jittery (a different kind of jittery than when I drank one), I would get anxious, my body would start sweating profusely for no reason, and I would get cranky. I started weaning off them when I realized that those things were happening when I didn't drink one, and I wouldn't feel that way if I did drink one. It took me a few months, but eventually I was able to leave them altogether.
The reason why I started drinking them, and a problem that I had when I stopped, was that I often needed something to keep me awake on my drive home from work after an 18-hour day. I quite far from any of the places where I work, about 53 miles on average, with San Jose being 75 miles away. As I said earlier, I need something to keep me awake on the drive home, especially after a really long day, or when I get off work in the middle of the night. As an alternative to Monster, I've started drinking a water-soluble powder that they sell at Costco, Zip Fizz, along with an Argentine herb-based supplement, Bedoyecta, and sunflower seeds. I find that the combination of those keeps me awake long enough to drive home safely, it doesn't give me jitters, it's not habit-forming, and it doesn't keep me awake 3 hours after getting home.