Should you use opt-in pop-up forms on your website? Numerous case studies from big companies show that opt-in pop-ups, welcome mats and slide-ups help you grow your list. So this should be a no-brainer, right? Right, opt-in interstitials will add subscribers to your list. But at what cost? Find out what those case studies that praise opt-in interstitials aren't telling you to make sure you (and your business) can afford to make this sacrifice.