The success rate varies from person to person, but it was high enough for the FDA to approve it for chronic Migraine, not headache. Continue reading this article for more information; FDA Approves Botox for Migraine.
It is really important to have an accurate diagnosis as to what kind of Migraine or headache disorder you have. This is for purposes of treatment, learning about them and being able to get care if you're away from home. You can read more about this in The Type of Migraine Does Matter.