A bicycle lock is only part of the solution.

by Dirk de Fietser
(Saint Paul, MN, US)

The best designed and manufactured bicycle lock is nothing more than an inconvenience to the dedicated and determined thief.
What is worse, most locks that are obtainable here in the US are at best an unpleasant and aesthetically challenging inconvenience to the rider.
Its main objective is (somewhat) immobilizing the prized bicycle when not applied to its intended use, so as to inconvenience possible thieves.

After that, that same lock becomes a hefty nuisance that needs to be safely stored and/or lugged around until the next time it needs to be applied to its purpose.
More often than not it will compromise the look and feel of the bicycle or challenge the rider by having to store it somewhere and cart it around with him.

That being the case and having established out of experience that a determined thief will find ways to defeat even the best lock, why not go with the solution that I have seen in Europe.

A decent lock is mounted and fixed to the frame of the bicycle and locks the frame to the rear wheel (optionally also the front wheel.).
Optionally there is a self-storing steel cable that can be slung around an immovable object of choice and is operated by the same lock.
These registered locks have a unique number and are sold with a one year (and renewable at very reasonable rate) theft insurance tied to the unique number of the lock, covering the purchase price of the bike as well as the lock.

When the bicycle is operated, the key remains fixed and inserted into the lock, but once locked the key is released.
If the locked bicycle is stolen anyway, then the key and its issued with purchase spare (Both labeled as: "Do Not Copy") are proof to the insurance that the lock was actually used and you can expect to be re-embursed for its loss.
When found afterwards, the original owner can be easily notified and possibly re-united with his/her beloved iron steed.

Just my $ 0.02