I too found Mag-Citrate to help with bad headaches. I've notice a major decrease in my regular day-to-day headaches, that are normally brought on by scents. Oddly, since I've been taking Mag-Citrate, I'm no longer sensitive to a wide range of scents that I used to be allergic too... I'm now almost convinced that it's the Mag-Citrate that is making the difference. I find because of the reduction in headaches, I have more energy and am more able to keep up with my regular activities.
In respect to helping fibromyalgia, I'm not really sure. My fibro is in remission at the moment, tho I do get occasional pains that are reminiscent to the old fibro days. I haven't had many of those pains since taking the Mag-Citrate (it's been about 6 months), but I can't say for sure that it's the mag-citrate that is helping... What I can say tho, is that I won't stop taking the Mag-Citrate to find out.