I did work for a guy who said caffeine had beneficial affects. He said dosage and the individual was the key. He knew quite a bit scientifically and actually had a canister of "pure caffeine" in powder form (it wasn't coke).
A couple years ago I developed a complete intolerance to caffeine. A single cup of coffee or can of coke will ruin my whole day, making me feel like sh*t in a squirrely hang-over type fashion. The weird part is I drank one or two cups a day for years with no issues. It took me more than a month to figure out why I felt like turd snacks every day. I cut out the coffee and boom, there was the problem. I still can enjoy a cup of decaf though.
Developed a similar issue, i drink decaf now, had a huge cup of it this morning actually. If i drink caffein now it makes me sweat pretty badly, it's no good. I use to drink energy drinks and diet coke among other things all the time for years.