Unfortunately in GB. We're not allowed to 'legally' cut a vehicles engine out and should we do it and our vehicle causes an accident the owner of the vehicle is liable to a greater arse shafting session than the scrote that stole it.
I had a Clifford with blackjax fitted to my escort cabrio a few years back and it was set up so as to flash the brake lights but not cut out the engine. I soon changed that.

since then I have been out to too many vehicles that have failed alarm immobiliser systems. As I spend a lot of time legally removing vehicles with alarms sounding and hazards flashing and rarely get challenged I cannot see the point in them.
There are very quick and easy ways of disabling most alarm sounders and flashing lights and the scrotes know how to do it.
If I had a choice between mechanical devices and alarms or immobilisers i'd take the mechanical systems every time. The only other thing I rekon is worthwhile is a tracker. The key to a tracker though is to not let people know you have one fitted.