They are worth it if you have the quids for it. Most companies are a fit and monitor monthly fee, I have come across few who are a fit and leave it job, so it racks up some cost, but, your insurance goes down a little, in essence you are only really out of pocket for the installation of the box, but then if your insurance is seriously low, then you end up paying over the odd's for it.
Something like the idea of this though looks quite handy: http://www.trackershop-uk.com/Pro-FleetMan+4+Wired+GPS+Tracker-p-56.html
One off fee, no monthly reports, and you can monitor your car yourself. The only thing I am unsure of, is if the police can sniff out the trackers signal in the event of a theft.