They are both true. Surveys of the U.S. population show that most people do not get the recommended daily intake of magnesium from what they eat and drink. This is particularly true of older men and adolescent girls. However, our bodies compensate for this to prevent symptoms of overt magnesium deficiency, such as seizures and abnormal heart rhythms.
Nevertheless, increasing magnesium intake to adequate levels in the diet or with some supplementation has cardiovascular benefits, such as modestly reducing blood pressure and reducing the risk of stroke. It may also help prevent hearing loss from excessive noise, migraine headaches, and menstrual pain, and it can improve insulin sensitivity in people with type 2 diabetes. This is discussed in the Magnesium Supplements Review
, which includes information about recommended intakes, dosage, and ConsumerLab's tests and comparisons of magnesium supplements.