I picked up my habit when I was in high school, because I was really struggling with sleep at the time, and caffeine actually helped me stay through the day, and sleep at a certain time at night. It doesn't have much of an effect other than keeping me awake unlike others have mentioned. As an adult now, I drink about 2-3 cups A DAY, dark roasted, and 10 ounces. I've also become a fan of Starbucks too.
To be quite honest, it's not really a bad habit unless you actually abuse it. If you drink on average 4+ cups of coffee or 400mg of caffeine or more, it is actually considered a drug, and you CAN get overdosed by it. I had a friend that's been there before. He vowed to not take any caffeine again and decided to stick with natural energy.