Bikes get stolen. Expensive looking bikes (especially ‘mountain bikes’) are more likely to get stolen than tatty ones.
Even a cheap lock is better than no lock – but….
The basics still apply. A cheap cable lock isn’t much use. Lock bikes to something secure – cast iron tenement banisters don’t count. (Put the lock over the wooden handrail. It usually has a metal base, and won’t shatter when kicked.)
An article in the current edition of the CTC magazine has lots of detailed advice and even more salutary tales.
It outlines most of the tricks of the bike theft trade – not so you can copy them, but to be aware of how to increase the chance that a potential thief will walk past your bike.
Even expensive D locks can be broken with the ‘right’ tools. The article (available on quickrelease.tv) points out that it’s best to ‘fill the space’ with as much as possible of bike, wheel and bike parking to deter/thwart the thief. “Look for bike racks that make such locking tactics easier. The best Sheffield stands are those in an ‘M’ shape not a ‘U’. Lock at the lowest point of the ‘M’.”