I think I've built up a bit of a resistance to it.
My usual day consists of:
Two cups of black instant coffee between 8:00 and 10:00, five or six cups of tea throughout the rest of the working day and a 500ml bottle of Diet Coke at lunch.
This keeps me ticking over while I'm at work, I don't get highs or lows from that.
However, if I break my routine and have a Redbull or Relentless at lunch instead of a Coke I'll feel a bit more wired. The same if I have a proper cup of coffee or two at the weekend.
I think I've only has too much coffee once — I went from buzzing to shaky to sick over the space of an hour or two.