A lot of health websites have data about caffeine. I particularly like one page that the Mayo clinic has, about how much caffeine is too much; their guidelines are that 200-300mg of caffeine aren't harmful for most people, but more than 500-600mg can negatively effect health. But they also remark: Some people are more sensitive to caffeine than are others., and also that How you react to caffeine may be determined in part by how much caffeine you're used to drinking.
This difference is due in large part to the fact that the body develops a tolerance for caffeine. But sensitivity to caffeine is also influenced by other factors, including sex, age, body mass, stress level, and other drug use.
From talking to my friends, I've seen a wide variety of responses to caffeine. Some people say it has little to no effect on them; other people find it really helps them concentrate, and they say they need it to get going in the morning. Some people avoid it because they find its effects unpleasant. Other people say they enjoy its effects but are able to function just fine without it.
How do I respond to caffeine?
My own response to caffeine is hard to generalize about because it is different on different days. In general, small amounts of caffeine tend to make me feel a little better: more focused, a little more positive and energetic. Too much caffeine can make me feel bad, though. Also, there are some ways where I respond very poorly to caffeine. If I'm a little tired, caffeine can perk me up, and if I'm a little hungry, it can stave off my hunger. But I've noticed that if I'm exhausted or ravenous, run down to a more extreme level, caffeine makes me feel terrible: it can make me feel even more sluggish, depressed, and just generally awful. Fortunately, I know these times now and I haven't had this problem for years because I avoid caffeine when I'm feeling tired or hungry beyond a certain threshold.
One reason that I prefer tea over coffee is that coffee has a bit too much caffeine for me. Coffee almost always makes me jittery, unless it's a weaker cup and I'm careful to drink it slowly (which results in lukewarm coffee, not the most pleasant thing). Tea, however, is gentler. Although the caffeine content of tea varies widely, it is almost always substantially more than tea. You can read more about how much caffeine is in various teas on RateTea's page about the caffeine content of tea.
The only problem I've had with too much caffeine from tea has been when I've been idly drinking cup after cup of tea in a Chinese restaurant, and I haven't been paying attention to how much I am drinking, and I have come out wired. Sometimes I also end up this way after tasting a bunch of teas, although I usually tend to space them out so this doesn't happen. But this feeling is usually not as unpleasant as the jitters I get from coffee, probably due to other chemicals in the tea.
I'm always able to function fine without caffeine. I'm a morning person and I wake up just fine, usually before my alarm on days when I have it set. My lowest point in the day is the late afternoon, and I often drink two or more cups of tea some time between noon and 6, so if there's any time of day when I have a noticeable caffeine addiction, it would be this time. But I wonder if it's really that or just that it's a natural low point in the day: I remember feeling this way from when I was a kid, long before I had any regular exposure to caffeine.
So, how do you respond to caffeine?
I would encourage you to either post comments or write your own blog post about caffeine. I think it is useful to see how different people respond to caffeine differently, and I'm curious to read what experiences others have.