I take magnesium for three reasons - migraines, endurance and bone health. Magnesium is vital to life, being part of every cell. It is even a critical part of energy supply, being needed for ATP usage. I believe it is a critical micronutrient for runners.
Only 32% of the US population gets their RDA of magnesium, and only 27% in NC . Magnesium levels in the blood fall after a marathon, which suggests that runners are more at risk for magnesium deficiency than sedentary people. There is even some suggestion that magnesium deficiency is related to the death of a marathon runner due to mitral valve prolapse. 
There is some evidence for magnesium supplementation for improving athletic performance. "That means that athletes wouldn't be able to work or train as long as they would if they had better magnesium levels" . There is evidence that magnesium deficiency can result in a significant reduction in exercise performance . There is increased loss of magnesium in athletes  through sweat and urine. It seems a reasonable assumption to this author that the increased sweating that occurs in an NC summer would exacerbate this loss.
Magnesium is a critical component of bones, making up about 1% of the structure. Lower levels of magnesium produce, more brittle bones with larger bone crystals . Inadequate magnesium results in lower blood calcium levels, resistance to hormone that control bone density and reduction of vitamin D effect; all result in reduced bone density . There is some evidence that magnesium supplementation on its own will help bone density . One study showed this in healthy older white subject, but not in black subjects . Magnesium is also believed to be critical to calcium absorption . (If you have concerns over bone density, don't forget to make sure you are getting enough Vitamin D.)
There is evidence for magnesium supplementation helping prevent migraines [2,3]. Personally I have seen a reduction in my migraines, and an improvement in my low light vision. If you suffer from migraines, I would strongly recommend reading the reference articles.
There is some very tenuous evidence that magnesium can help with muscle cramps . This is not a problem I suffer from, so I have no personal perspective.
Other Impacts of Deficiency
There are studies that have linked magnesium deficiency with asthma, emphysema, Attention deficit/hyperactivity disorder (ADHD), Depression, type 2 diabetes, heart disease, high blood pressure in women, and several other problems. [1, 16]
Magnesium, is absorbed at about 30-40% efficiency . Good sources of magnesium in food include nuts, cereals, coffee, green leafy vegetables and especially chocolate . Cheap supplements use magnesium oxide, which is very hard to absorb; I use magnesium orotate, though chelated forms are also supposed to work well. Avoid taking Magnesium sulfate as it is used to treat constipation . Vitamin C can help absorption of minerals. Therefore I take magnesium with Vitamin C on an empty stomach. (Bathing in Epson Salts can also raise magnesium levels in the blood.)
My research indicates that magnesium is generally a safe supplement at RDA type levels. Excess magnesium is filtered by the kidneys, so overdose is normally only an issue for people with kidney issues. Magnesium supplements can cause diarrhea, but this seems to be linked to the type of supplement. As with all my advice, verify it independently. If you have any doubts, consult your physician.
I buy my Magnesium Orotate from Lucky Vitamin as they seem to have the best price I have found - http://www.luckyvitamin.com/item/itemKey/55469. I generally take one tablet (500mg) with Vitamin C on rising, and another post run.
References - Magnesium
 Linus Pauling Institute - Magnesium
Magnesium deficiency (medicine)
 USDA intake figures for NC
 Magnesium in biology
 Epsom Salts
University of Maryland Medical Center - Magnesium
References - Magnesium and bones
Magnesium supplementation and osteoporosis
Magnesium intake from food and supplements is associated with bone mineral density in healthy older white subjects.
 Magnesium: A Key to Calcium Absorption
References - Magnesium and Migraines
 Magnesium Treatment for Migraines An Inexpensive but Equivocal Treatment
Are Migraine Headaches a Symptom of a Magnesium Deficiency?
References - Magnesium and endurance
 Lack Energy? Maybe It's Your Magnesium Level
 The dangers of magnesium deficiency in endurance athletes http://findarticles.com/p/articles/mi_m0FDL/is_4_14/ai_n24940334/
Micronutrients (magnesium, zinc, and copper): are mineral supplements needed for athletes?
 Iron, zinc and magnesium nutrition and athletic performance.
References - Magnesium and cramps
 Exertional Heat Cramps: Recovery and Return to Play