if you're talking about the 'old' endurance racing products (erp) master zapper, there were 2 versions. the 12v one that you left the motor in for 30 minutes (got hecka hot), and the ac one that takes just a second or so to saturate the magnet.
as far as usage for the ac version: take the arm out of the can, put a metal slug inside, line up the magnets in the can with the clamping pieces (make sure to get polarity correct). tighten the clamping pieces with an allen driver, and hit the switch for 2-3 seconds.
i'll reiterate it because it is the most important thing...make sure you have the polarity correct. the zapper is not strong enough to reverse the magnets. you'll ruin a motor if you don't do it correctly.
as far as value... i'd say in the neighborhood of about 175-200. team1rc sells an even more powerful ac version, but it runs 425 plus about another 40 for shipping. trinity will custom make you a permanent rare earth zapper for about 1200.
as far as it being any good...yes. it doesn't do anything to improve a new motor, but it'll gain you a bit back on your used motors. after zapping you can fell the arm spin harder, and you can also see mild changes on a dyno. i usually get about 1-3 watts, and 2-300 more rpm (peak) after zapping, but benefits are going to be determined by the condition of the motor going in.. just like a comm lathe, it's a good tool for maintaining top performance.