It's the age-old dilemma between marketing and user experience. We want to push our messages out to as many relevant people as possible via effective, available and innovative channels but not at the expense of the user experience. It's a fine balance between hitting your conversion metrics, trying to offer your visitors relevant and timely information whilst not disrupting their journey.
This is a great summary of what to consider when launching a new overlay so that you can get that balance right.
This combination of power and speed means it’s dangerously easy to launch one without much consideration for user experience. Thus, they’ve developed a bit of a reputation for being effective… and disruptive. But the disruptive nature of overlays is actually inherent to their effectiveness, because it focuses the visitor’s attention on a single offer. They eliminate the paradox of choice and present the visitor with a simple yes or no question.