I have good news for a lot of you out there: coffee is not just good for you, it's great for you. In fact, it's not too much of a stretch to call it a legitimate anti-aging tonic according to the latest research. Think that's an exagerration? Well, ready below and judge for yourself.
So, yes, there is such a thing as a healthy addiction. Dark Chocolate and green tea are other examples. Marijuana, smoking and fast food are all addictions that will kill you, but coffee according to the latest research is a true friend. And let's face it: it's hard to find a healthy friendship that will last through the decades.
CAUTION: Coffee and caffeine have their scary side. For more information, read about The Potential Dangers of Coffee and Caffeine.
1. All Cause and Cardiovascular Mortality. Coffee is one of those rare factors that can actually reduce your risk of dying. The reason: it is close to neutral or maybe slightly positive with regards to cancers deaths but significantly reduces cardiovascular death rates. For example, a 2008 study followed over a 100,000 men and women for 18 years and found a clear dose dependent decrease in all cause (overall) and cardiovascular mortality.  Remember: dose dependence is the gold standard of such studies and adds more weight to the evidence, because it meant that the more coffee that was consumed the less the risk of dying. Previous studies had found the same thing by the way. NOTE: Decaf coffee showed the same results but to a lesser degree. 
2. Cancer Reduction. The mortality studies mentioned above do not show significant overall cancer reduction among coffee drinkers. However, coffee does appear to decrease the risk of certain specific types of cancer, including cancer of the colon, kidney and non-melanoma skin.  Still other studies have shown that coffee offers signficant protection against prostate cancer, especially the most agressive kind.  And, interestingly enough, the same benefits were found for decaffeinated coffee, indicating the results came from coffee's phytochemicals and not the caffeine itself.
3) Diabetes. Another mystery of coffee consumption is the fact that it seems to, in the short term, negatively affect glucose metabolism and insulin sensitivity yet very significantly reduces the risk of diabetes according to multiple studies. In fact, those who drank seven or more cups per day had only one half the risk. 
4) Weight Loss and Weight Maintenance. Coffee, both through caffeine and some of its consituent ingredient, has been shown to help keep those pounds off. 
5) Dementia and Alzheimer's Reduction. Several studies have shown that coffee reduces the risk of dementia and Alzheimer's, unlike tea.  3-5 cups/day confers optimal protection according to one study. In fairness, one study did not find such a reduction.  However, the reason is likely the dose as yet another study found that too much coffee resulted in a loss in cognitive gains.  Again, right around 3 cups appears to be the sweet spot.
6) Alertness and Mental Performance. It's no secret that coffee boosts your mental sharpness, endurance and acuity. The studies show alertness and mental performance are enhanced with coffee consumption.  It may not turn you into Einstein, but it can give you a decided edge. One study found a dose dependent response where the more coffee that was consumed, the better the cognitive performance.  NOTE: Decaf coffee did not have nearly the same benefit.
7) Exercise and Athletic Performance. Coffee boosts lipolysis, the ability of your body to burn fats and rates of nerve impulse transmission.  This means greater endurance and, therefore, performance in most exercise and athletic conditions. One study of middle distance runners showed that it improved overall race time, final stretch run speed and VO2.  What else is there, eh? By the way, coffee does this without exclusively relying on caffeine's epeniphrine-boosting properties. For example, one study showed that coffee actually dampened the effects of epinephrine. 
8) Reduced Blood Pressure. This one has researchers scratching their collective heads. Some initial studies showed that coffee increased blood pressure and thus the risk of hypertension. However, follow-up work showed clearly that heavy, "chronic" coffee drinkers actually had lower blood pressure.  The threshhold is about 5 cups/day and has the strongest correlation in males.
9) Parkinson's Disease. A 2002 meta-analysis shows "strong epidemiological evidence that smokers and coffee drinkers have a lower risk of Parkinson's disease".  (This is just about the only disease where smoking is protective.) The reduction in risk for 3 cups/day is in the range of 25-30%. A previous study from a few years prior had arrived at the same conclusion. 
10) No Fluoride. This is just an apologetic argument to use with your tea-drinking friends. Green tea is similar to coffee in the sense that it is a caffeinated beverage that strongly reduces the risk of cardiovascular disease, some cancers and all cause mortality. However, green tea has a big disadvantage: an abundance of flouride. Flouride can be hard on the brain and your precious neurons. Black tea has a lot of flouride and green tea double that.
11. Prostate Cancer. One recent study found that heavy coffee drinkers, decaf or regular, had a 20% reduction in all kinds of prostate cancer and a 60% reduction in the most aggressive form of prostate cancer. 
NOTE: Tell your woman about the wonders of coffee as well. Most of the benefits above apply to her as well!
REFERENCES:1) Annals of Internal Medicine, June 17, 2008, 148(12):904-914, "The Relationship of Coffee Consumption with Mortality"
2) J Natl Cancer Inst, 1986 May, 76(5):823-31, "Coffee drinking, mortality, and cancer incidence: results from a Norwegian prospective study"
4) The Lancet, 22 February 2003 , 361(9358):702 - 703, "Coffee consumption and risk of type 2 diabetes mellitus"
5) American Journal of Clinical Nutrition, October 2006, 84(4):682-693, "Coffee, diabetes, and weight control"
6) Journal Journal of Alzheimer's Disease, 2009, 16(1), "Midlife Coffee and Tea Drinking and the Risk of Late-Life Dementia: A Population-Based CAIDE Study"
7) Am J Clin Nutr, 2009, 90(3):640-646, "Coffee drinking in middle age is not associated with cognitive performance in old age"
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9) Neuropsychobiology, 1993, 27(4), "Investigation of the Effects of Coffee on Alertness and Performance during the Day and Night"
10) PSYCHOPHARMACOLOGY, 110(1-2):45-52, "Does caffeine intake enhance absolute levels of cognitive performance?"
11) Medicine & Science in Sports and Exercise, Fall 1978, 10(3), "Effects of caffeine ingestion on metabolism and exercise performance"
12) Br J Sports Med, 1992, 26:116-120, "Effect of caffeinated coffee on running speed, respiratory factors, blood lactate and perceived exertion during 1500-m treadmill running"
13) J Appl Physiol, Sep 1998, 85(3):883-889, "Metabolic and exercise endurance effects of coffee and caffeine ingestion"
14) Journal of Hypertension, Jun 1990, 8(5), "Association between habitual coffee consumption and blood pressure levels"
15) Annals of Neurology, Sep 2002, 52(3):276-284, "A meta-analysis of coffee drinking, cigarette smoking, and the risk of Parkinson's disease"
16) JAMA, 2000, 283:2674-2679, "Association of Coffee and Caffeine Intake With the Risk of Parkinson Disease"
17) http://esciencenews.com/articles/2011/05/17/ coffee.may.reduce.risk.lethal.prostate.cancer.men