I've had them (rarely) and they are killers. I wish I knew what set them off so I could obviously avoid them, but I have no idea. Could be something you eat or drink, or some tension related.
I have found that gentle massage of the temples and of the back of the neck (at the base of the head) helps.
I know some people who get them regularly and I, sure as hell, am glad I'm not one of them.
