I soon stopped, because I think they aren't worth it (which made me the world's worst salesperson. I remember near the end talking one old woman out of it.) The first important thing is that they won't prevent the nightmare encounter where a thief enters the house when you're there. Second, police are too swamped to have ever reacted in time to catch the thief. To my knowledge, it has never, ever happened.
There is a possible benefit that it may truncate the time in the house. To me this is not a big issue, I suspect most competent thieves are efficient with time.
Finally, there is an unobservable deterrence effect, but that is easily obtained by faking the appearance of a system without needing to enter a contract and pay monitoring fees. The most useless thing about security systems, the monitoring, is the only thing that costs serious money.
The final benefit is life-alert style stuff, but there are better solutions for that.
On the other hand, there is a massive placebo effect, a sense of safety that the monitoring provides. I suppose that is what people pay for.