Coffee drinkers, would you say that drinking coffee makes you, say, 15% more productive for the hour after you drink it? If you gain any productivity benefit, is it due to something about drinking coffee, or something about not not having had coffee?

I would estimate at least 20-30% more productive for at least two hours. If I am especially tired before, it could be double that. This is added benefit from drinking coffee; I only started about a year ago, and my productivity shot up. If I don't drink coffee, it goes back to 'normal' levels.