Complete scam.
I was moving and called 3 times to have my service address changed--the person on the phone couldn't ever figure out how to schedule it, and no one ever called me back. I had another company set up my service at the new house and called ADT to cancel. She was very nasty and informed me that there would be a charge of $376 for canceling my contract early. I told her the only reason I canceled is because they wouldn't return my calls to have the service moved.
I just realized that they have continued to charge my credit card for the past 3 months. I called to cancel AGAIN, and the guy tells me it's going to be $376.