It would be hard to find someone, who is using some of these zappers to get a feedback. I am just looking on a pictures, and for me they seems not very high-quality built (maybe i'm wrong, but that is the impression).
For me this one looks much better (overall quality feeling, the box, the PCB, etc):

http://cgi.ebay.com/Z4eX-EXTREME-3-Frequency-Hulda-Clark-Zapper_W0QQitemZ3500...

or even

http://cgi.ebay.com/SPRING-SALE-HANDS-FREE-HULDA-CLARK-PARASITE-ZAPPER_W0QQit...

or of course you can try the cheapest $10 device at

http://tendollarzapper.com/

if it is just to see what a zapper is and is it worth your attention.

And last, but not least, Zapper is a public-domain technology and you can always do it yourself following some of the freely available schematics.