Synesthesia is real but is not considered a disorder; it's merely a condition (like being 'left-handed'). Occurrence of Synesthesia varies from 0.005% to more than 5% of a population on a scale of 'pronounced absolutely' to 'subconscious/unconscious awareness' respectively.
http://en.wikipedia.org/wiki/Synesthesia
http://www.synaesthesia.com/ (tests available here)
http://www.synesthesiaworld.com (Blog with Synesthete's Interviews, Synesthesia Test with Videos, etc)