wireless security systems/video intercom

I have wireless ADT.

Love it.

Just be sure to replace the batteries regularly.

One time I accidentally tripped the alarm, and the phone was ringing from the ADT office literally within 15 seconds (which, when the damn siren is blaring, feels like an eternity).

I have been using Alarm.com monitored service for nearly 4 years now. I am very pleased with the service. When I was looking for a vendor, I was reluctant to choose one that required a hard line telephone (ADT required it back then, not sure whether they still do)- partially because that is too easy to disable and partially because I had a hard time justifying paying for a land line just for the alarm system (I am a cellular only household). The monitored service performed just like NYCMatt descripted with ADT- I have accidentally tripped the alarm and they were calling me within seconds. They are now including internet video monitoring if you have the cameras with the monitoring service ( I don't so I can't report back). I know they have an iPhone app but it's not yet enabled for the Blackberry Storm (damn)- not sure about Android.

I installed the system with a friend who used to install car alarms when we were teenagers- so this was second nature to him- very easy. The system also has a reporting system- so it alerts you if a battery is low or if there are other issues with a sensor. In one case- the main module failed (it's G.E. equipment)- and Alarm.com service emailed me immediately, and provided terrific customer service while we worked through the problem.

My building has an intercom system- and several of us would eventually like to upgrade to a video intercom. We aren't ready to make that investment yet- but I too am curious to hear thoughts from others about the various systems.