A girl at our yard swears by it. As it is mainly based on magnesium, it will only have an affect if your horse is deficient in magnesium (many are). I wouldn't get a branded source of it though as it is incredibly expensive. I would just get a bag of magnesium, I use these guys http://www.naturalhorsesupplies.co.uk/p/category/0802204936-Magnesium+Supplements/
Lots of info on dosing, what type to get etc.
someone I know gave it to their horse at the recommended dosage and their horse was much calmer for about a week - then it turned into mental crazy horse!
Not sure if you can overdose on it, but do be careful!