I am a big fan of exit interviews. I have learned more doing exit interviews than most other management techniques. When people are on their way out and have no fear of saying exactly what they think, you can learn a lot.
It is rare for an investor/VC to do exit interviews. I only do them in situations where there seems to be a significant problem in a portfolio company and I want to get to the bottom of it.
But if you are the CEO of a company, you should be doing exit interviews with everyone who leaves your company until your company gets to the point that it is impossible to do that. Once you pass that point, your senior team should be doing them along with you.
Here's what I like to do.
First, get a sense from the exiting employee's manager what the cause of departure was. Get the manager's take on the situation. Context is very helpful in situations like this.
Second, don't make an exit interview a witch hunt. Make it a conversation about the good and bad things about the company, the job, the people, etc. The less confrontational the exit intereview is, the more you can learn.
Finally, don't take everything that is said as gospel. There are always two sides to every situation. I like to understand both sides as well as I can. Everyone has an opinion and an agenda and its best to understand everything in that context.
Doing exit interviews is a lot like doing references. The patterns that emerge over multiple interviews are the most telling and that is what you want to be listening for. Exit interviews are a great way to get those patterns out on the table where you can see them.