I have the lightkeeper, but to be honest with you, 2 years ago I purchased a device called a buzz box off of Planetchristmas and love it for any incan work.
You plug the black box in to the wall, plug a light string in to the buzz box and press the big red button. Either the string will light, or it will buzz where the problem is!
It cost me about 150, but considering I can find an issue with a string in about 10 seconds, it was well worth it!!!
Mike.