I question the effectivness of an alarm. I understand that we all want to keep our cars from being stolen or burgled but who really responds to an alarm? If I was walking down the street and I saw some guy (or girl) at a car and the alarm was going off I wouldn't stop, for two reasons. One, it could be the owner forgetting to turn the alarm off or two the theif/burgler could knife/shoot/bash me. Or if I was lucky enough to overpower the person they can turn around and charge me with assault or sue me. If you went to the cops and told them an alarm was going off they would most likely ignore you or turn up 3 hours after the event. IMO alarms only deter amateur crims, if somebody REALLY wants your car they'll work out a way past the most sophisticated alarm/immobiliser that money can buy. A flashing light on the dash and some no name alarm stickers from Dick Smith is probably the best value for money deterrent.
At the Summenats a couple of years ago I heard about a guy that had a GT Falcon (XA I think) that had an alarm, immobiliser, club lock and he parked near security guards and the car was stolen in broad daylight without anyone seeing or hearing a thing.