All test equipment performance relies on the skill of the operator.I have been in the electrical trade for 30 years and the no contact testers are one of the most valuable tools I own. Common sense being most important. The testers can tell you much with proper interpretation. Given there are limitations.
They will show a hot even if the is no neutral. They will sometimes give a false positive due to inductance. They will not read through wet romex. (This condition is fully disclosed as a warning with the tester I currently use). They can fail just as any other battery operated device.
I always test on a known live circuit or rub the tester briskly on my sleeve to see if it is working.
I have used many types of these testers and have settled on the one osha uses. In my opinion it is the best.
There is no switch to malfunction and no external metal parts.The tester I use is a santronics.
http://www.santronicsinc.com/ These testers are also very tuff.
I also second the warning posted by chrsb since the voltstick is made of metal.
