Cyclists have a touching faith in apparently robust bike locks. This pic (snapped on a Cambridge street yesterday morning) may persuade them to think differently. It shows how easily an apparently thick chain can be neatly snipped with bolt cutters. The work of a moment, I’d guess.
LATER: I had a nice email from Dermot Ryan, who knows a thing or two about this stuff.
The type of lock in your picture, which is a cable lock rather than a chain, is famous among experienced cyclists for being easily defeated. A bolt cutters, as you suggest, can dispatch one in seconds, but some of the cheaper versions can be bust open by simply pulling vigorously on the bicycle.
Suffice to say, one can put one’s faith to a large extent in robust bicycle locks made out of tempered steel (for example), but not cheap cable locks.