Ken --
Sorry that I can't give you stats on this but here's our take.
When a website is working well, we'd be scared out of our heads to add pop-ups ... why annoy people who are already buying?
On the other hand, I wouldn't hesitate to experiment with them on a site that was a dead dog ... in the hopes of breathing some life into it.
Personally, I cross off sites with pop-ups -- no more visits from me (unless they have something that absolutely nobody else has ... and that is very rare!)
-- Phil
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