I have never conducted or been subjected to an exit interview, so this is 100% speculative...
I think how honest the departing employee will be in an exit interview will depend on why they're leaving.
If it's because of an unrelated life change, then they're probably honest.
If it's because of a problem with the company, then they probably aren't -- I know that when I've left jobs because of a problem with the company, I would never tell them that because I wouldn't want it to affect what they say when potential employers call them. I'd probably just say it was because of an unrelated life change of some sort.