They have variable length contracts from 1 to 3 years (for most alarm companies, shortest is 3 years), AND, you own the equipment after the contract period (which is relatively insignificant anyway as far as price of an overall average system).
The system is easily self-installed, so no holes in walls, no invasion into your home by others during install, no mess, etc. Also, the system is easily portable if you move. And even though it's an all-in-one solution, you can hide it out of the way and use remote keypads with it, which gives the system I have a very professional polish (the control panel I just hide out of the way).
I have ADT for three years now and I good with its services. Sometimes there are false alarms but ADT is great in providing its services to its customers. Their outstanding customer service + fair & honest pricing: They answer quickly, they don't try to push unnecessary product, and they are transparent in ALL their pricing, from contracts to sensors.