I have never employed a private investigator, and I hope I never have to do it, because real investigators have nothing to do with fiction and movies investigators. In fact, they don't solve exciting crime cases, but most of their cases are related with adulteries and little tricks. So, I would prefer not to need them.
For the same reason, I think I wouldn't like to be a private investigator, I wouldn't be able to wait for hours to someone meeting with his lover in a bar or with his financial consultant in a bank.