When we bought our house, it already had a security system in place. It had been installed by Power Security Systems and was monitored by them.
The system seemed to have some issues with Aliant Internet - it would beep at night when it would have trouble contacting "home base." Once we switched to Eastlink, that problem disappeared. But the company were no help with the issue.
Their actual office is only open in the morning, so it is often hard to get in touch with them to make appointments, etc.
The monitoring is fantastic and very affordable. I am going to be calling them soon to add additional features to our system and will add a new review/update my review then.