When I bought my car, I hesitated between Lojack and the BMW alarm (about the same price ~$500). I went with Lojack primarily because I think car alarms are useless and because my insurance deduction (through AAA) was $150 annually. I have had the car for 4 years now and Lojack has paid itself.
The idea that, in case of a theft, you would rather have the insurance money than your car back is only valid if the car is very recent. My car is 4 years old, worth about 20K and I am still paying for it. If I needed to replace it with the insurance money minus the remaining of the loan, I would have to add quite a bit to buy a new or similar one. If my car was stolen, I would rather get it back and and get it fixed.