Tracking someones moves covertly

GPS Tracking Device http://www.thinkgeek.com/gadgets/security/8212/?cpg=cj

With this device you can track someones every move covertly. Is this a good thing? How many of you think that the benifits of these things out weigh the negatives? Would you use it on someone? What situations warrant such an invasion of privacy? Personally I think we need countermeasures.