Understand that pretty much all locks are easily defeated by professional thieves- so do0n't leave your good bikes on the street or anywhere else where you can not see them or where a criminal will have access to them. Realize that locks are primarily to keep regular people honest; they will stop a non-professional thief who might be tempted to just walk off with an unlocked bike- but I am not aware of any lock which will stop or even slow-down much a real thief who has a few basic tools.

The only time a lock is impenetrable is when it's yours and you forget/lose the key. Then, miraculously, no one can open it