Cobra Trak5 is what I have, Thatcham 5, remote immobilisation etc etc. No loom issues with a GT-R, and very sensitive (roll 10 meters and your phone is ringing and the text alerts are coming in). Those that were not bothered about the loom went for the cheaper Tracker Horizon and Response and seem happy. I don't think anybody gets insured without one from the amount of discussion they generate.
Not really sure if it's the best thing in the world to have one but if the car is going to go, I guess I'd rather minimise the time the scum spend with it.