We had an ADT system for about 4 yrs, and recently changed to a contract with Monitronics. Yes, it's a 24 month contract, with many of the same provisions that ADT had, at a considerably lower price.
Because we already had the same basic wireless system installed, we got a ton of upgrades at no cost. Things like an extra motion sensor, smoke detector that ties into the alarm system, cell phone system, etc. Should our home phone line go out, or the wires get cut, the alarm automatically calls over the cell system.
Our city, in an attempt to control the costs of responding to false alarms, implemented a policy this year, where they won't respond to an "unverified alarm event"...so, if you aren't home and they can't contacy you via cell phone, or are tied up by the "bad guys", they won't come out, even though your alarm is blaring...
The Monitronics system we had installed gives more contact options, including 2-way phone/intercom service thru the alarm panel. On top of getting all the freebies, by having the smoke detector tied into the system, we got a 10% discount on our homeowners premiums...