I have over 20 years experiance in the alarm business. ADT is a nationwide company with local district offices. The great thing about ADT is as a Tyco International owned company they have great buying power and very good training and support availabilty. The down side is that ADT is only as good as the local office, therefore making a blanket thumbs up or thumbs down statement about them would likely not be accurate. The service from the ADT operation in Birmingham, Alabama may be surpurb, whereas the performance reputation for ADT in Seattle may be very poor.
My overall experiance with ADT in the residential market has been positive. If I needed a higher end commercial system, I'd go with it's sister Tyco owned company, SimplexGrinnell.