If I had only had one of these, my beloved TREK 850 wouldn’t have been stolen way back in 1998, in San Francisco: Even there, bike thieves don’t carry bolt cutters of appropriate strength (and size), I reckon.
seen on Bikemap’s Facebook site
So, put it onto your wish list for Santa, if you have a commuter bike you like.
And if you are strong enough to carry the lock…
If you don’t want to go through the extra pain of carrying such a lock, go for the most lightweight prevention of theft there is: Always find a place to park your bike right next to one that is much more attractive and expensive (and not very well locked, either).
Lightweight and cheap means of bike protection: a parking space next to a much more attractive and more expensive bike (note that it is only the old one, the one with the rather special map holder, that got chained by its owner, in this case, though …)
Another example how the smart cyclists can save the pain of carrying a lock but should be on the pretty safe side, still.