Along with my lithium one*, I also recently finished & analyzed a long blinded & randomized self-experiment using magnesium citrate: http://www.gwern.net/Nootropics#citrate
The results are a bit complex, but it appears that initially the magnesium citrate helped me a lot, but then the benefit shrinks over time and becomes severely negative. Combined with the high dose I was taking and the relatively long half-life of magnesium, I infer I wound up overdosing. I plan to run a second self-experiment with a much smaller dose to see if I can get the good without the bad.