The pros and cons of ZP have been debated in several lengthy threads in the past few years. Basically, a lot of people like them, but the people who understand electronic circuits pretty much agree that the ZP device does not put out as strong a signal as most other zappers, such as those derived from Clark's original and later designs. Parazapper (admittedly a competitor), put some test images up on his site:
There was a thread sometime in the last year about someone else cranking out a $10 unit which had a stronger output, but I couldn't find it. Probably because the thread started with an unrelated subject line. Maybe someone else remembers it.