User Experience First for Modals, Pop-Ups, and Overlays

User Experience First for Modals, Pop-Ups, and Overlays
Rand Fishkin of Moz
_There are three or four metrics that I'd urge you to check out to compare whether this is doing the right thing. Those are:

Bounce rate
Browse rate
Return visitor rates, meaning the percentage of people who come back to your site again and again, and
Time on site after the element appears

So those four will help tell you whether you are truly interfering badly with user experience._

https://moz.com/blog/popups-seo-whiteboard-friday