Forum Moderators: Robert Charlton & goodroi
If they DO have useful content, then the site may suffer, as the link to the pop-up may be javascript or otherwise ignored by Google, and links from the pop-up may not get spidered.
Straight ad pop-ups probably don't impact on the serps at all, they just get squashed by blockers, or incentivize people never to visit again.
Unless your target audience is total saddoes, then there's almost certainly a better way to monetize your site. Saddoes rarely convert, they just go "Oooh a pop-up. Isn't it amazing what they can do"