I think that caffiene is REALLY understated as an addictive and mood altering drug. I know a guy who is completely addicted to coffee and he gets really fucked up off it to the point where he can't think straight or form complete sentences (cuz he is buzzing and rushing so hard) and then he comes down off it and he gets aggitated and depressed, so he takes another hit and he's buzzing again for another few hours. But he always crashes and ends up feeling like crap. Most of his problems with using too much caffiene are mental/emotional, he's a complete wreck for most of the day, either coming up or coming down from his favourite recreational drug. As a result of all this caffiene use he has problems with a rapid heartbeat and chronic anxiety. The biggest issue with it is that he gets into this state where he hates himself and the world and he dispairs about everything. He does this at about 4:30 in the afternoon each day, just as he is coming down off his 1:30 fix. It's like clockwork, you could literally set your watch by his erratic mood swings and his overwhelming urges to babbling incoherently like an idiot about nothing (when he is peaking).
I've tried caffiene in high doses (coffee), and I was even addicted for a while, and it wasn't fun. I don't like being physically addicted to anything. It's fun for a little while, but the negative side effects were just too much for me to take. It made me depressed and pissed off for nothing and I had really bad anxiety (just the physical sensation of anxiety, I wasn't actually mentally anxious, I just felt that way). So anyway, I gave up coffee, and I haven't touched the stuff since.
I've done a handful of different drugs in my life. One drug I've done a number of time is speed. From doing both speed and coffee (at different times of course) I can say that they are far more alike than unalike. In moderate amounts of speed and caffiene, speed is a little more psychedelic, but I find caffiene far more stressful and addictive. The times I took speed, I would be happy and talkative for a whole night, and then the next day I would be ok, just feeling a bit tired but that's all. The times when I've been buzzed on coffee for a day, the next day I woke up *craving* more coffee like the way a nic addict craves tobacco. Perhaps if I took enough speed over a long enough time I would've developed a craving for that as well, but I never did so I don't know. Speed doesn't have that action which makes you irritated, angry, and depressed either (I've never seen it have that effect on anyone actually, unlike coffee).
I think that caffiene is good in small amounts, but in large amounts, like in strong coffee, it's fucked. I have a nick-name for coffee: liquid stress. Some people can handle it though I guess, just like some people can go out and drink a bunch of booze and wake up the next morning fresh as a daisy. I'm not like that though.
If you are addicted to coffee, ask yourself: Do you get angry/moody/depressed in the evening after you come down from the high? If the answer is yes, then caffiene is probably a negative thing in your life, and reducing your intake would help your mental state and overall happiness a great deal. If you are nervous and stressed all the time, coffee is likely the cause of that too, and that is sooooo unhealthy to be in that fight or flight state all the time, it'll kill you, it'll sap your energy and throw your immune system out of whack, etc. Stress kills, literally, it leads to illness, and coffee is liquid stress.
I'm not anti-caffiene entirely, just anti-overdosing on the stuff. I think that mild tea can provide a little kick/buzz which can help your concentrate and work at something, and that's a good thing, but getting fucking ripped off several coffees in a single day just isn't healthy, and it contributes greatly to that sense of dispair and angst that is indicative of the human condition (as well as creating untold health problems).
Satori
PS: The stuff I've said here, I'm not just making it up, I very close with a person who's life's work it is to study how drugs affect health and psychological well-being. Coffee's first action is to make you happy, and it's second action is to make you pissed off and depressed, and very few people are immune to this second action.