If someone wants it bad enough, the bike will be stolen. You just gotta get something that will be enough of a deterrent based upon where you keep your bike and what kind of neighborhood you live in.
I bought a Xena XX6 disc lock and park my bike behind my car in my garage with the forks locked. For 80 bucks, that little alarm gives me a lot of piece of mind though... that's for sure (since I don't have theft insurance).
When I first bought the alarm, I was trying to take the battery tab out... and the damn thing went off... I'm surprised my ears didn't start bleeding. It is insanely loud.