Beware. Beware I say.
While not a worry if you've had a great experience at a company, love your coworkers, enjoyed every day and have literally nothing to complain about. Then, enjoy the process and gush all you want. Nice words pave the way should you ever want to come back to said company.
The challenge comes if you have NOT had a great experience, did not love your coworkers, did not enjoy the place and have lots to complain about. How do you provide constructive feedback to the company without using the opportunity to vent and rave about everything you are leaving over? Fine line, folks.
Here's a personal example. I was production manager at an agency when they suddenly lost the account. Rumor was most of us would be absorbed into the new agency, but a lot of us went searching for new jobs just in case. I landed a job, gave my notice and had an exit interview on my last day.
I wanted to do them a favor while they began to hire a replacement for me. I proceeded to tell them that I was too senior for the job and that, when they rehire, they should find someone with less experience. A junior person would be perfect for the kind of printing they were doing, which wasn't terribly complex. They kept listening, so I kept talking.
Fast forward to day one at my new job. Which I h.a.t.e.d.
Telling you all the reasons why I hated this new job on day #1 would be an entire blog post unto itself, so I will spare you. Suffice to say, I immediately called my old boss and asked for my job back. And surprisingly wondered why their answer was no. Huh.
This is a tame example of what topics you should probably not get into in an exit interview. As well, save your rants on specific people or find a way to get your point across without being disparaging. These meetings, while they appear confidential, aren't always so.
It's important that a company know what cogs might be broken and exit interviews allow the forum to get those points out in the open. I'd just caution you to really think about what you want to say and why.
If you truly would like to come back to the company some day, say so. If you truly would never like to step foot on their premises again, then really think twice about what you choose to talk about. It's a fine line between trying to help them get better, trying to vindicate any wrongs you suffered while there, and maintaining your reputation as a professional.