I've had one fitted to a car Karey. When it got nicked the police found it dumped down a country lane within 30 mins of me reporting it missing. Superb but expensive as it needs to be fitted somewhere not easily found then there's an annual subscription to pay!
Every traffic cop car has the responder fitted to find your bike/car if it goes awol and on the flip side your insurance will be quite a bit cheaper if you have one fitted