I'm a police dispatcher, and personally I would never pay for a monitored alarm. First of all, I'd say well over 95% of the residential alarms we get are false. It's more common for commercial alarms to be actual break-ins, but the overwhelming majority of those are false, too. Residential burglar alarms are one of the most common calls that we get, and in my two and a half years working there, I could probably count on one hand how many of those I've seen that were actual. Obviously it depends on where you live, the crime rate, and all that jazz, but in my experience it's very rare to see an alarm actually catch a burglar.
And even if it is actual, it's insanely rare that the police actually show up and find somebody still in the house. I can only think of one time that that happened while I was working, and that was at a business. Here's how these things work. The alarm goes off, and the monitoring company gets the signal. They usually try to call on premise, and if they don't get an answer there, they start going down their contact list trying to get a keyholder. Only after they've done all that do they call the police. That usually takes at the very least 5 minutes, sometimes more like 15 or 20. Once we get the call, assuming there's an officer available, it takes an average of around 5 minutes for the officer to get there. If it's a busy night, there may not be anybody available, and the call will just have to hold until there's an officer to take it. So you're typically looking at an average of at least 10 minutes before the police show up. More than enough time for a burglar to get the hell out of Dodge. The cops show up, but they can't do anything but stand around and say "Yup, somebody stole your shit."
In my opinion, it might be worth it to have an alarm that makes noise, but isn't monitored by anybody. The noise would scare any potential burglars away, because how are they to know that the alarm isn't monitored? That way you don't have to pay the monthly fee, or worry about getting fined or harassed by the police department if and when your cat starts setting off your alarm.
But anyway, that's just my 2 cents, from my experience being part of the whole process. But it's just personal preference... If you've got the money, it really couldn't hurt, but I wouldn't pay for that service, because IMHO it's just not worth it.