This topic comes up often on both the Cobra and Corvette forums.
The bottomline seems to be that any alarm system will not deter the professional thief. Ask yourself how many times you heard a car alarm go off, and see no one move/react. These kinds of alarms pretty much help to prevent kids from "joy riding" your car. Other types of devices, besides electronic ones, that one can easily implement include hidden igition and/or fuel switches, hidden remote battery disconnect switch, etc... Again, these won't stop the pro who will just take the car to a secure location, then figure out how to start it up (or just cut it up, or export it.) But they will help prevent "joy riders".
The best devices that can be purchased appear to be "thief reaction". Lo-Jack and even On-Star work really well with stolen car recovery. They're expensive, however, and they do not deter the actual theft. Some pro's can even disable these systems, but not all can.
Truly the best theft prevention is you, and what you do. Yes, a simple kill switch will prevent the "joy riders" - this is always a good idea, and maybe the PATS system is sufficient here. But, just like going into a large city, your best defense is awareness of your surrounding.
Park where you can see your car, or where with little effort you can get up to see your car - and check it, often. Don't valet park it. Park it where it there's a lot of foot traffic - it's easier to fix a little scratch or "door ding" than it is to loose it.
When at home, park it your garage, and leave your garage door closed - leaving your garage door open (with your car inside) is like advertising. Lock your garage door. Park your daily driver in front of your garage door. When going on vacation (or like absence), tell your neighbors that you're gone.
Last, make sure that your insurance is paid up and in good standing. It's truly painfull to loose a car that's loved - can you imagine how you'd feel if you thought your insurance was paid up, and it wasn't!
Hope this helps...