I used to think a cable lock was good enough if I was only going to be, say, running into a store for a moment. Then I had to cut a decent cable lock off my girlfriend's bike because she'd lost the key...
People, using SMALL bolt cutters--small enough to be put in an inside pocket, jeans back pocket or a purse or knapsack--it took literally no muscle and one snip and it was cut through. 2 seconds, max.
Like cutting paper with a pair of scissors! Someone could have done it in the time it took me walk three steps toward the store and turn and look and back.
I now have one of those Abus folding locks. Heavy, yes, but less so than a chain and more versatile than a U-lock; folds up into a nice little carrier that fits on my seat tube, unobtrusive and slick as hell. Kinda expensive, but worth it.
