I have a Xena, but like others have stated it is more of a deterrent than anything else. I also have an actual alarm as well, so my hope is having both sirens going off would prevent most thieves from bothering.
Also as others have said -- More than anything it eases my mind about people messing with my bike (aka, touching it, trying to sit on it for pictures, etc). I figure once the alarms start sounding they'd rethink their new FB profile pic lol.