I wake up every morning and take my Vitamin D with a smile on my face and a song in my heart. Exagerration? A little but not much. First of all, Vitamin D is the Muscle Vitamin. That's right - muscle cells have Vitamin D receptors and low levels can decrease muscle strength and performance.
Vitamin D is also the Great Forgiver. Let's face it - none of us guys have been perfect with our health: we've all abused our bodies and pushed our luck in one way or another. Vitamin D is one of our best insurance policies against our inability to be perfectly disciplined. It lets us forgive and forget. (Exercise is the same way. Remember: "exercise covers a multitude of sins".)
NOTE: Be sure to read my link on Boosting Your Testosterone with Vitamin D.
Why do I say this? The beauty of Vitamin D is that it dramatically improves outcomes for what I call the Big Three Killers of us males: heart disease, cancer and autoimmune disorders. In fact, let's face it: two of the scariest things about middle age are the idea of a sudden heart attack dramatically taking away our life or a cancer and/or autoimmune disease slowly and painfully doing the same. This is where Vitamin D comes to the rescue.
In the last five years study after study has uncovered Vitamin D's almost magical and uniquitous powers over our health. It regulates and is involved in over 2000 genes. Almost every system and part of our bodies are affected by Vitamin D levels. And the beauty of Vitamin D is that it is a cheap and readily available supplement.
Think about this: one of the stressors in life is to stay in good health and keep from dying. Seriously, if you can stave off cancer, diabetes and heart disease, then you very likely have decades of productive and enjoyable life ahead of you.
CAUTION: Recent research has shown that there is likely a rather narrow band where Vitamin D provides benefits and, furthermore, shows that high Vitamin D may actually be harmful. Check out my page on The Potential Dangers of High Vitamin D for more information.
1. Heart Disease. Vitamin D deficiency has been oft associated with cardiovascular disease  and the reason is likely that it seem to affect arterial inflammation levels.
3. Brain. "Maintain your brain". That's a good motto if ever we've heard one and Vitamin D is one major key to holding onto your memory and ability to learn. Read about Vitamin D and the Brain.
4. Flus, Colds and Immunity. As it turns out, low levels of Vitamin D were recently found in one study to be linked with a significantly greater incidence of colds and flus. See this link on Immunity for details.
5. Autoimmune Disorders including Diabetes. The studies look very promising and show that higher levels of Vitamin D may be strongly protective against diabetes - yet another major killer of us males.   Crohn's disease, multiple sclerosis  and rheumatoid arthritis  also seem to be linked to lower Vitamin D levels as well. Again, this is probably because of Vitamin D's centrality to immune function.
6. Bone Health. Calcium gets all the glory, but Vitamin D is every bit as important for bone health. Why? Calcium is regulated by Vitamin D.
7. Diabetes. Keep your Vitamin D levels up. One recent study showed that 24% of study participants with low Vitamin D levels developed diabetes by ten years afterward, a very high percentage.  Another study found that those with the lowest Vitamin D levels had a 47% greater chance of developing prediabes. 
8. Testosterone. Some people consider Vitamin D a hormone - that's how powerful it is. Regardless as to what you call it, it can also profoundly affect our hormone levels, including testosterone. For more information, read my link on Boosting Your Testosterone with Vitamin D.
9. Nitric Oxide and Blood Pressure. Getting your Vitamin D levels out of the basement may actually increase your arterial nitrix oxide levels, something I discuss in my page on Nitric Oxide and Vitamin D.
You may have read that sometimes it is a deficiency of Vitamin D that actually causes issues and so this may not seem to be an issue for you. Unfortunately, it is actually difficult to get adequate levels of Vitamin D for urban dwellers for many reasons and for this reason one study found that three fourths of adult and younger Americans are deficient for example.  Here's some of the top reasons why:
So how much should you take? Almost every export now agrees that the old RDA's (200 IU for those < 50 and 400 IU for those > 50) are not enough.  Many nasty diseases and overall mortality is clearly associated with a Vitamin D deficiency below about 15 ng/ml, measured by blood levels of 25(OH)D or serum 25-hyroxyvitamin D, and the current post-age-50 RDA will put you right in this neighborhood. However, a 2010 study, for example, found improvements up to 43 ng/ml in "death, diabetes, cardiovascular disease, myocardial infarction, heart failure, high blood pressure, depression, and kidney failure."  There are other studies as well that show substantial benefits above the old 15 ng/ml level.
So how much Vitamin D would you have to take to get the maxiumum benefit mentioned above of 43 ng/ml? Well, this is where it gets interesting: 43 ng/ml corresponds for 25(OH)D to 107 nmol/l. And to get your plasma levels up to this level, you would need to take about 4000 IU of Vitamin D per day, according to one recent study. 
This is why you will see almost all experts now recommending at least 1000 IU per day and perhaps even more. Always talk to your doc, but I take 1200 IU per day, especially since I avoid the sun and put on sun screen.
1) Circulation, 2008, 117:503-511, "Vitamin D Deficiency and Risk of Cardiovascular Disease"
2) J. Nutr, Feb 2005, 135:323-325, "Symposium: Vitamin D Insufficiency: A Significant Risk Factor in Chronic Diseases and Potential Disease-Specific Biomarkers of Vitamin D Sufficiency"
3) Ann Rheum Dis, 2007, 66:1137-1142 "Vitamin D and autoimmunity: new aetiological and therapeutic considerations"
4) Archives of Internal Medicine, 2009, 169(6):626-632, "Demographic Differences and Trends of Vitamin D Insufficiency in the US Population, 1988-2004"
5) Environ Health Perspect, 2009, 117:A196-A196, "Vitamin D Regulates MS Gene"
7) WebMD, "Are You Getting Enough Vitamin D?"
9) American Journal of Clinical Nutrition, May 1999, 69(5):842-856, "Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety"
10) WCIR 2010; Abstract, "Prospective risk of hyperglycemia in a South Florida population with low levels of vitamin D", http://www.medpagetoday.com/MeetingCoverage/WCIR/23202
11) Men's Health, Jul/Aug 2011, p. 33