Love it! Been using it for 2 years now. Works like a charm. Excellent if you need to step out to get a cup of coffee, or to go to the bathroom, etc.
Whenever I am in a new place I demo the siren before I step out for the first time so that everybody within 60 to a 100 feet clearly gets the message. And it works great, putting the fear of God into anyone around, because my MBPs have NEVER been stolen, nor has the siren even been 'accidentally' set off.