I would think a casual theif would be deterred by about any lock, definately by any kryptonite brand lock, and really definately by a U type. I view the casual theif as one who wants the bike for himself, or to sell for $20 for some crack. A professional theif certainly knows the ways to lock a bike as well as any of us do. The pro will go for what's quick and marketable.

I've learned a lot in this thread and by looking over the locks on krypto's site. They are not all created equal. If I ever get a real bike, one you guys wouldn't laugh at, I'll certainly get a very good U bolt and a 2nd lock of some sort. Or not park outdoors.