I tried them but they were expensive.
The other thing that makes a difference is the Navtrak ADR that I have on my car. It alerts Navtrak if the car moves without a credit card sized chip being in the car. Its good because the system works if the car has been stolen with its own keys.
I have driven off several times leaving my wallet at home! Navtrak call me to check the car hasnt been stolen - if it has, they tell the Police where it is.
In the UK, you cant get insurance without some kind of tracking system but it seems the ADR system really gets the quote down.