The over-use of caffeine is bad for you, and it's really easy to over-use.
There, I've said it. And here is a John Hopkins Bayview Medical Center's report and Psychiatric Times article to agree with me. Please note the line about "...caffeine-induced sleep disorder" in the Psychiatric Times.
It's also important to note that caffeine alter a person's mood or behavior can be seen in as little caffeine as 100mg. That's 15mg less then is in a Tall (the smallest size) Starbucks Café Au Lait. The US average of caffeine consumption is 280mg/day, according to a different John Hopkins study.
Yes, this is from a person who used to drink 96oz of Mountain Dew in an eight hour work shift. I feel loads better now that I'm not drinking caffeine then I ever did while I was drinking it.
(Crossposted to my personal journal.)