Caffeine - the legal running performance enhancer
Most runners who race, or perform hard workouts, benefit significantly from caffeine consumption.
When I tell people this, it's either obvious, or mind blowing. Caffeine will enhance your running performance. And it is legal, one of the few performance enhancers that is not banned by the World Anti-Doping Agency. Not that most runners have to concern themselves with this.
The benefits only appear to those performing aerobic exercise though, that is to say that short term, high intensity activities do not receive a benefit (e.g. sprinting).
Caffeine is actually so effective that some experts, like Dr. Mark Tarnopolsky of McMaster University in Canada say, “There is so much data on this that it’s unbelievable, it’s just unequivocal that caffeine improves performance. It’s been shown in well-respected labs in multiple places around the world.” 
In one study, elite runners improved their time in a treadmill run to exhaustion by 1.9% with caffeine. Caffeine boosted time to exhaustion in a cycling test by 15 minutes in another study. And in a study involving swimmers, caffeine was found to enhance performance in maximal-effort swims of up to 25 minutes' duration. All aerobic exercise -- all solid improvement.
This isn't cheating, it's just a smart way to get a little bit extra for that goal race you've been training for.
But it's a diuretic, right?
You're right, it is, but it's very mild. So mild in fact that exercise essentially mitigates any effects.
In a study to evaluate the amount of excessive sweating in long distance running, 9 athletes were given 450mg of caffeine either with 30 minutes of exercise or without. The running resulted in a decrease in the urine flow and also a decrease in the amount of caffeine that was excreted in the urine. This supports the belief that caffeine should not pose a threat of urinary problems to endurance athletes. (Barnard, 1992, via )
How to Consume it
Runners commonly take caffeine in pill form (proven to be more effective than equal amounts of caffeine consumed in coffee) 30 to 60 minutes before races to enhance competitive performance. What's the optimal amount? The ergogenic effect of caffeine is dose-dependent.
The maximum effect is seen with doses of 5 to 6 milligrams per kilogram of body weight. For a 150-lb runner this translates to roughly 340 to 400 mg, or the amount of caffeine you'd get in 14 to 17 oz. of drip brewed coffee. The minimum amount of caffeine the average runner must consume for a measurable ergogenic effect is about 2 mg per kilogram of body weight.
It makes less sense to use caffeine as a daily workout performance enhancer, for two reasons. First, workouts are seldom maximal efforts, and the rationale for caffeine supplementation is to enhance maximal performance. Second, the ergogenic effects of caffeine consumption decrease with habituation. For this reason, if you are a regular coffee drinker, you should cease coffee consumption four to six days before participating in a race. 
Trial and Error
Like all new things, it's best to try out your caffeinated powers in a workout, as opposed to finding out on race day whether being hyped up on caffeine is right for you. Also, it should not be used as a sleep substitute or a crutch for low energy.
I have experimented with varying doses and can vouch for the fact that too much will leave you feeling incredibly "hyped" up, and unable to settle into a pace, but finding the right amount produces worthwhile results.
If you're looking to experiment, to get a little more out of your next workout or race, caffeine is a safe and proven consideration. Thoughts?