I've got two of those cheap Chinese alarms (to match my cheap Chinese bikes) I think they are a blatant copy of a better known alarm. It's a little on the sensitive side so on very windy days I turn them off. As for a thief just tearing it out that depends where on your bike it is to remove mine they would have to remove the seat and the airbox and the damage done to the loom (I've wired in the immobiliser too) to get it out would mean they ain't going anywhere unless they push it all the way.
They give me peace of mind when I park in crappy places so I think they are worth it.
