I'm a cable lock guy too. I carry two of them in my top case. They're good for a ton of stuff. I can lock my panniers to the bike with them, lock my helmet (through the opening for the face) and jacket to the bike, or even lock the bike to a stationary object. I do have one of those mini u-bolt locks too, that I lock around the front rotor to keep the wheel from turning.
And no, none of these are theft-proof, but they'll deter the casual thief, and they may slow down the professional one. I never leave my helmet or jacket on the bike. A career in law enforcement taught me that a thief will steal anything, even if he has no idea what it is, if he thinks he can sell it to someone else. And some people are like cats; they'll throw your helmet across a parking lot just to hear the sound it makes when it hits the ground.
