After reading Armageddon's post, it prompted me to put in my 2 bits.
You need to be dinking AT LEAST one gallon of water if you're either taking in high protein or are on the juice. Coffee doesn't count towards this. And, it has diuretic qualities - very COUNTERPRODUCTIVE
Your body can get acclimated to the "thermogenic" effects of caffeine. That's why people who drink a ton of coffee or caffeined soft drinks aren't affected much by the caffeine.
I shy away from any drinks with caffeine. I always take ECA prior to workout and ALWAYS get a buzz (25mg ephedrine, 200mg caffeine and 1 non-coated aspirin, 325mg- i think). Makes me sweat like a pig and increase my heart rate.
I stongly believe that if I drank lots of caffeined drinks, I wouldn't get this effect.