I described part of what caffeine does to me in the WCDT thread, but I'm also the recipient of what appears to be a family curse. There's an amino acid in coffee (I forget the name) that burns a small amount of iron in the blood. It's not related to the caffeine, either - it's in decaf, as well.
My system over-reacts to this amino acid. The same happened to two of my maternal uncles, and my maternal grandfather (my mother is unaffected, it seems to be a gender-biased reaction). But it is definitely hereditary.
After drinking two cups of coffee (OK, two 10-12 oz mugs), I would get pale and pass out in the afternoons. I had not drank coffee regularly before college, this started my junior year when I started drink it more regularly. I went to give blood, and the stick was so low in iron that the nurse asked me, "Why are you still conscious?"
A visit to the doctor, and a long talk with my mother confirmed my condition. More than an 8 oz cup of coffee a day = anemia, plain and simple, even with iron supplements.
And so, I am a tea-totaller. My favorite (but I haven't seen it for years) was something called "Morning Thunder", which had a good strong blend of black tea and chickory, along with some cinnamon and vanilla flavors. The caffeine level was somewhere between an avereage cup of coffee and an average single shot of espresso...
Just right!