I had a car stolen last week (Golf v6 4motion), by someone who came to look at it to buy it. I didnt tell him it had RAC Trackstar on it when he asked about alarms. Police got car back after 40 mins - and collected the people from inside the house it was outside. It ended up costing me, about 250 quid (towing fee, broken mirror, and imobiliser recoding since the key wasnt recovered), and then my wife made me sell car to a dealer (which cost about 1-1.5k less then selling privately)
I didnt need to have trackstar/tracker, and I wish I didnt. I would have been better off if it hadnt been recovered :-( Very sad, but true.
Now, I know RAC Trackstar uses a gsm (mobile) data connection to constantly report back the gps location of the vehicle. IE, excellent. I beleive that Tracker Horizon, uses the pager network -
I suspect trackstar will work out of the UK, Tracker certainly wont.
But, you do have to think whether you want the car back .... (as Flip mentions)