A modern (not like the one in the movies usually) lie detector which is conducted by a PROFESSIONAL (not just some Joe who took a week long course) is VERY VERY reliable.
In fact, from my experience, people who don't want to take them, are usually lieing.
Secondly, that stuff from the movie that was mentioned is bull$hit. That is hollywood crap. It is very VERY VERY hard to trick a lie detector, assuming you don't have a serious mental complication.
In fact, modern professionally administrated lie detectors are so accurate that the mossad, the shaback, and the CIA feel confident enough to use it on THEIR OWN AGENTS to verify truthfulness.
Don't believe anything in the movies.
Whether or not it should be allowed in court is a more complicated and seperate discussion, but as far as I'm concerned, if you're innocent, and really want to prove it, take one. If you're innocent you'll pass.