The value of exit interviews is directly correlated to what you will do with the information, and the number of interviews you do. If you have a low number of folks you want to do these on, the value of the information will be less valuable and more influenced by individual bias. If you don’t do anything with the information, or don’t trust the reliability of the interviewee, why bother???
I have seen these used in the past by clients to focus on areas where they suspect problems – the usual ones of training, supervision issues, orientation. The biggest help they have gained is in revising the orientation and training processes. The most helpful questions have been "How would you change the training/orientation process? What did we not teach you that you now realize would have been incredibly helpful?"
Believe it or not, many clients don’t ask questions about the departing employee’s new wages, hours, and benefits provided by their new employer; these clients utilize other methods to ensure a competitive place in the marketplace.