Caffeine does weird things to me. Sometimes it only seems to make me more sluggish and tired than before, and makes me want to go to sleep. I also get this nasty itchy feeling in the eyes, a bit like the one when one has been sleepy for a long time, and a general feeling of being unwell and not thinking straight. For some reason this comes especially with strong black tea and most strong energy drinks like Red Bull. If I have too much caffeine and it's stretched over a longer period, the sleepiness doesn't go, but I can't sleep when I try. The itchiness gets worse, and I have this tension inside that makes me restless without being alert. Nothing nice really.
On the other hand, some drinks like Pepsi-Max or one drink we have that has lots of guarana as well as taurine and caffeine, do make me more hyper and alert. Maybe it's due to their other ingredients more than the caffeine itself.
Other foods do make me very cheerful and energetic, almost manic - black chocolate with mint filling that has real mint oil, and crystallized ginger sweets. It could be a very bipolar thing, though. Now I'm thankful to one friend of mine with bipolar who suggested the mint chocolate thing (her husband is a food chemist and knows what foods can balance out or affect one's mood); it's been a great help when I'm feeling down.