i don't really think they will ever stop pro theives. if someone really wants your car that badly, they will bring a car trailer and some tuff looking dudes. that said, an alarm might stop someone looking for a quick joyride or to steal your change.
i bought mine from Tweakit at mona vale, it has remote start, central locking, immobilser, in-built turbo timer, sms tracking optional add-on and instructions in Chinglish: about $220-$250. i had to install it myself.
if you are getting it to reduce insurance premium check its on the list of approved alarms.