This app works as advertised for me.
Unfortunately in addition to app alerts the service also sends an email to you for every incident, so if you have the app youll see every incident twice--once as an app notification and once in email.
I contacted the developer about this, and he provided a workaround by resetting my email address as non-validated so that emails to me would be suppressed and my only notifications would be via app notifications.
AlpacaLips about FlashAlert Messenger