I am very much a disbeliever in Intelligent Design.
ID requires an intelligent designer. However, how is this possible? If the universe, by definition, contains everything that exists, then that includes the intelligent designer. Where did they come from? Create themselves out of nothing (which apparently could not have possibly applied to the universe, necessarily a less-complex entity than the designer)? Are they eternal beings (again, eternity couldn't apply to the universe)?
ID proponents throw out ludicrous-sounding figures about the probability of certain occurrances happening without design. Yet, improbable things happen. How probable was my birth? That my parents should happen to meet, that their parents should meet, etc? Consider the possibility that the universe is eternal. If time is infinite, then the probability of a given occurance at any point in time is 1. In that case, not only do apparently improbable things happen - they MUST happen, no matter how improbable they seem. That's only one idea, of course.
But most of all, I don't believe in ID because I've never seen objective evidence for it. All I've seen is alleged lack of evidence for other theories used in lieu.
Also, see
The watchmaker argument refuted and I've found reading transcripts of Victor Stenger's talks to be interesting also.
What should we teach in schools? ID is too socially significant to ignore. As far as giving it equal time goes, I'm not too sure about that. Teachers should be able to point out (perceived) advantages and deficiencies of ID and evolution.