Drs Carolyn Dean (author of The Magnesium Miracle) and Mildred Seelig (author of The Magnesium Factor) both say that magnesium deficiency is very common in modern cultures and also that blood tests for it are not reliable. And they both say that different people require different daily amounts. Dr. Dean, for example, takes 3 times the official RDA. There is a free audio download (called Magnesium and Our Health) of Dr. Dean discussing magnesium at http://drcarolyndean.com/natural-health-resource.
Quite a lot of it is about bone health and osteoporosis. At http://drcarolyndean.com/resources.html
you can see which supplements she recommends. I know that Dr. Dean herself takes Angstrom Liquid Magnesium so thatís what Iím taking and I find it more potent than other supplements Iíve tried.