How does long-term use of caffeine affect productivity?



I am trying to figure out whether caffeine helps productivity in the long run. Looking back 10 years from now, how much more/less productive will I have been if I were to drink coffee every day, or every second day?

Reviewing what has been written on the topic by our community so far:

  • Justin summarizes effects of caffeine: impairment of long-term memory, narrowed focus, increased short-term memory and recall, increased attentional control, increased memory retention and retrieval. From this, he tentatively concludes in favor of use for tasks that benefit from these effects. However, does this conclusion still hold for regular use? We need to take into account reduced stimulation due to increase in tolerance and potentially impairment during withdrawal.
  • In a comment on Justin's article, simplyeric writes: "There are studies (that I read years ago, and have no link to) that show that consistency is better... that consistent low-level caffeine drinkers are more alert than their non-caffeine colleagues, but less jittery than high-caffeine people (optimum seemed to be 2-3 cups per day)." This is the kind of evidence I am interested in. Does anyone recall such studies?
  • Gwern's review of nootropics lists a number of potential negative effects, including effects on memory, performance (in high doses), sleep, and mood. In justifying its use despite these effects, he states that "[his] problems tend to be more about akrasia and energy and not getting things done, so even if a stimulant comes with a little cost to long-term memory, it's still useful for [him]". Is there conclusive evidence that in the long run, caffeine increases energy and helps with akrasia?
  • Skatche's review of psychoactive drugs presents anecdotal evidence in favor: "Taken on a fairly regular daily schedule, caffeine seems to improve my attention, motivation and energy level." To what extent do such anecdotes reflect true improvement? If regular use of caffeine were to result in decreased baseline performance and if the effect of caffeine were limited to restoring baseline, this could feel similar from the inside.
Edit: Studies pointed out by siodine suggest that caffeine has few or no beneficial long-term effects:
If caffeine was consumed, the adverse effects of lowered alertness and headache were avoided, but even after 100+150 mg of caffeine their alertness was not raised above the level of alertness showed by nonconsumers of caffeine (group N) who received placebo (Figure 1, middle panel). This result is similar to that from an early study comparing responses to caffeine of coffee drinkers and abstainers (Goldstein et al, 1969), and is consistent with the claim, supported by a variety of subsequent findings, that regular caffeine consumption provides little or no net benefit for alertness or performance on tests of vigilance (James and Rogers, 2005; Sigmon et al, 2009).
The study also demonstrated robust acute effects of caffeine unconfounded by caffeine withdrawal, but no evidence for net beneficial effects of daily caffeine administration.
Overall, there is little evidence of caffeine having beneficial effects on performance or mood under conditions of long-term caffeine use vs abstinence. Although modest acute effects may occur following initial use, tolerance to these effects appears to develop in the context of habitual use of the drug.