I took the cvsa a few months ago.  It wasn't bad.  My interviewer told me that the machine won't tell him if I'm lying , just if there's stress in my voice, and if there was then we would talk about it afterwards and try to figure out why there would be stress in my voice.  In my opinion, the interview is designed to make you nervous if you have something to hide.  If you have nothing to hide then don't sweat it.  I had read some people's horror stories on this sight about polygraph exams so I was really nervous going in to the interview.  Bottom line is that the cvsa machine is no better than a coin toss but if you are hiding something the interviewer may notice your nervousness if they're good.