I have been using FrontPoint for 2 years now and they have been great. The capability of the system is quite good, and it is easy to buy and add new sensors, etc - anything that is compatible with GE Simon XT. The system has apps for the smart phone, so you can monitor and control it from anywhere. You can set up notifications for non-alarm events also - I have one that reminds me if the garage door has been open for too long. They even have geofencing.
The customer service is superb. They are always patient and knowledgeable. They have helped me through installs of various sensors, and troubleshooting, and through a move from one home to another.