A few years ago, my parents had locked their bikes (mom had a Specialized hybrid, dad had his Bike Friday) to the fence, along with about 20 other bikes. When they came back with their veggies, every bike was gone except for my day's Bike Friday. Again, the cable lock was left, cleanly cut. We assumed that my day's bike was too custom and unique to profit from easily.
Coming back to these four bikes today, I noticed that two of the bikes were locked with crappy $20 cable locks. Remember that my mom's $50 cable lock had been sliced like butter. The lock companies even rate these locks as a 2/10. My mom's was about a 6 or 7.
The bike below is the one that really caught my eye.
While this final bike is MUCH more secure than the first there, Cyclicious has brought up a great point: the rear wheel is easier to steal and more expensive to replace. Check out his video here (thanks, @schwankytown for the head's up!): http://www.cyclelicio.us/2013/lock-rear-bicycle-wheel/
*Original post starts here: This final bike, a pretty Linus with Velo Orange accessories, had actually been locked by someone who wants it to still be there when they return. They have both their front wheel and the frame u-locked to the bike rack.
Of course, if a thief has enough determination and time, they can get through just about anything. Don't be the one with the crappiest lock that is the easiest to steal.