It is an approved technology by the US government. It is mostly used as an interrogation tool as opposed to a lie detector. Having only worked with interrogators I only know second hand how it is used, from my understanding it usually used in a good cop/bad cop scenario (such as, "The machine is saying you are lying, let me help you out and tell me what happened.") I am sure there much more knowledgeable people then myself on this forum. Short answer: as a lie detector it is unreliable,but as a interrogation tool fairly effective.
Pretty much. They've been sold as lie detectors based on the notion that stress == lies, this is obviously incorrect. The same notion is also what make voice stress analysis unreliable as well.
I think they're considered not reliable enough to count as a testimony in court for sure. That's not to say they don't have uses in other places. The intelligence community uses them quite often. The most common use most folks see is their application in getting and maintaining security clearances. They're part of a larger screening process in that case.