It's always seemed ironic to me that, the lighter the bike, the heavier the lock has to be--because the more expensive the bike, the more attractive to thieves. I heard a guy in a bike shop say to a guy who had just bought a carbon fiber bike and was asking for a lock, "A bike of that quality can't be safely locked--if you insist on buying a lock for that bike, I won't sell you the bike." A little (!) controlling on his part, but we get the point. I use an OnGuard U-lock with a cable for the wheels, but even with that, I never leave my bike locked on the street unless I'm within view of it. LIke others in this thread, I leave a heavy lock in the bike cage in the garage at my office, so I can commute unencumbererd.