The only time we ever got robbed was when we had a home security system. Cancelled that thing as soon as we got out of the contract. Not a deterrent at all.
In our previous home, we had a break-in while we were out of town. I believe the alarm system acted as a deterrent for any theft or vandalism. What appears to have happened is this: bad guy broke in through the basement window (right at kicking level). We had open/close sensors on the windows, but not glass breaks. So no alarm at this point. It looks like the bad guy went upstairs and opened the rear slider (presumably to let his fellow crooks in). Rear slider is on a sensor, so at that point, the time-based warning alarm went off (not the "real" alarm, the quiet one that lets you disarm the system before real one goes off). I believe the thieves heard this and immediately left. (They were nice enough to close the rear slider so the cats didn't get out!) But since the entry/exit delay wasn't ended by disarming, the real alarm went off, and the cops were called.
That was the first break-in I've experienced, and definitely felt violated. Nothing was stolen, and, other than the window, the only damage was the baby gate to the basement was busted off the wall. It's not my stuff I care about, it's the idea that an unsavory person is willing to break a window to intrude into my home.
It seems like a video monitoring system is useful just for seeing if you can see who stole your stuff after the fact. I haven't met anyone who was able to benefit from this, i.e. got their stuff back due to CCTV.
Another anecdote: my friend and his dad run a small business. Their office neighborhood had been experiencing a string of break-ins and thefts. So his dad bought one of those motion-activated cameras ($100ish), and set it up to send text/email alerts. No monitoring service, just auto-notification of all family members. Shortly after he set that up, my friend's brother got a text message along with a picture in the middle of the night of the thieves in action. He promptly called the police, who showed up and busted the criminals.
A side benefit of cameras: dealing with delivery services. I can't remember where I saw this now, but I read an account of someone dealing with FedEx or UPS. Basically, the delivery service was giving him the runaround, unwilling to believe his side of the story, until he said, "Look, I have motion-activated cameras, and the footage clearly shows that this is what happened." He said that the tone of the representative changed immediately, to one of complete and total accommodation.
I wouldn't call this story alone justification for a camera system, but it's a nice perk if you're going down that road. Even a fake camera on your front door might help keep the delivery guys in line.