Not surprisingly it depends! I just learned about this the other day, so I'm excited to share- and interestingly it makes a stronger argument for tea for some people.

There are slow metabolizers of caffeine and fast metabloizers of caffeine.

Caffeine in and of its self is a stimulant, looking at just mental properties yes it improves mental performance, namely alertness (related to productivity, concentration etc). However consider there is a body attached to the brain, when caffeine intake is too much people get jittery and loose focus. For some people, believe it or not, caffeine gives a really brief burst of energy then makes you tired . The difference in the two is how they metabolize.

Both fast and slow metabolizers absorb at the same rate theoretically. So you get this response of caffine blocking adenosine from binding receptors (adenosine binding receptors makes you tired). If that's blocked you get wakefulness. The body responds by creating more adenosine receptors to reach a homeostatic balance.

In fast metablizers you are very rapidly left with an excess number of adenosine receptors and the caffine is not there to block any of them, leaving you with a net increase of adenosine receptors, more places for the molecule to bind so you get more tired.

In slow metabloizers you taper off the amount of caffeine slowly so the body has more time to adapt homeostatically. Thus you don't feel tired, but more awake.

The interesting thing is if you have tea, green tea especially there is theanine in it. This actually allows people to slow the rate of absorption of caffine. So for fast metabloizers it is like a front end modification that somewhat allows you to mimic the homeostasis slow metabolizers feel. Your peak levels don't get as high, so you don't get such a robust response, but you are continuing to absorb caffeine.

For a lot of fast metabolizers this really increases ones concentration and thus productivity.

In fact caffeine plus theanine is one suggested treatment for ADHD.