ill chime in and say.. you can have an alarm and it wont stop a theif...
subs, amp, avic screen & brain, two headrest Tv's stolen @ school night time. parked on the MAIN asile, under light right next to the parking pay meter.. alarm on the car, xv791
i was out of range from the pager, so i didnt get the beep when it was broken into..
time away from car: 30 mins..
so even with a good alarm (pro install everything hidden where it cant be reached including pain generators) anyone that wants your stuff will take it