I use a cheap cable lock for my expensive commuter bikes, but I only ride them between home and work where there is secure parking. We have two bike racks at work. One is in an underground parking garage which can only be accessed with a badge and has a security camera right above the bike rack. The other is behind the building which also has a security camera and is patrolled by building security officers. If I want to run errands downtown, I'll use the beater bike and lock it with a beefy lock in an area with high foot-traffic.