Has anyone used exit intent popups?

Yes - in ecommerce I see them quite a lot. And they can be very effective (if done properly you should see at least 2-3x more 'churned visitors' returning to become customers within thirty days).

The big challenge here is that you have to not only give the person an incentive to sign up their email address, but you also have to convince them to change what they intended to do (leave your site) and instead stick around to enter the email address.

As I said above, in ecommerce this is pretty easy - just dangle a gift voucher (works better than a discount % code) in front of the visitor's eyes and, chances are, they'll drop in their email address. Because who knows whether they'll want to use it at some time in the future?

Outside of ecommerce though, it's much more difficult. A visitor's objection will be more complex than for ecommerce, and there are more potential objections (some very subjective). You don't need to educate someone or make them 'problem aware' for them to want to buy a dress from your ecommerce store - but you probably do if you're selling a CRM SaaS product, for example.

So because the objection is hardly ever price (although it often looks that way superficially) and it is really hard to guess what the real objection is, your exit pop-up will probably not perform too well unless you give the kind of crazy discounts that anybody would sign up for (eg 50% off for life). But you probably don't want to do that.

They work. As long as the incentive makes sense.

As opposed to a direct exit pop up, you can use exit surveys that help you figure out what’s missing on that specific page.

From there, you can add the missing information or address the problems on the page. It has the same effect.