Not very. In fact, in most courts, it is not considered credible. It's power is mostly in the person's belief that it has more credibility than it does. All the system does in measure GSR (galvanic skin resistance), respiration depth and frequency, skin temp and blood pressure. And rise or slowing in any of these can be associated with any emotion at all, and emotion does not indicate guilt of anything. Therefore, a 'lie detector' relies heavily on the guesswork (or 'experience') of the examiner.