I had my exit interview on the second-to-last day I worked there. The HR flunkie had originally booked 30 minutes but I told him he'd better book an hour in a conference room. He got a little scared, he told me when we began our interview, because he thought I, a 23- year employee, might go a little nuts in some rant. I didn't, but I did have plenty to say.
In the interview, I made it clear that my issues were not with my immediate boss(es) or my coworkers. Instead, my issues were with corporate policies (about group health eligibility for part-timers, and about telecommuting) and the commute I so much despised, especially after the company relocated to New Jersey.
I wasn't expecting any policy changes or some desperate plea to not resign, but the exit interview was somewhat cathartic.