Before you take the plunge and add one of these devices, have a quick read of the info provided in the following links. I believe it's from a reputable source and food for thought. Personally I don't use a grounding probe, but do use GFCI devices on all equipment hooked up to my system.
http://avdil.gtri.gatech.edu/RCM/RCM...ingProbes.html
http://avdil.gtri.gatech.edu/RCM/RCM...alDetails.html