How does arnica montana work? Is it a good homeopathic option?

With right use, yes. Homeopathic arnica helps especially in trauma & pain with bruising -- like contusions, sprains, and even surgical trauma. Research with face lift patients showed significantly less bruising: http://archfaci.Jamanetwork.Com/article.Aspx?Articleid=480929 when correctly matched to the person's symptoms, remedies work by stimulating innate restorative systems. Arnica can help in concussions too.