Coffee gives a +4 circumstance bonus to all saves for staying awake, alertness, and reaction for 1d6 - cups drank. After the Positive effects of coffee wear off the drinker of the coffee incurs a -2 circumstance penalty to all the saves mentioned above for 1d4 + number of cups drank.
Coffee as I'm sure everyone in the Police or the Military will tell you is an important part of daily (or nightly) life. However, it can eventually mess with you with conditions such as "Adrenal Burnout", ulsters and so on. Still, one thing I never liked was standing watch in a game, a warm fire at my back and not being able to brew some coffee while my fellows slept. Seriously, there is magic in troves in fantasy rpgs and no one has found coffee!