To make sure about the 'defective' tubular tumbler theory - I tried to open two locks from two different makers, and I succeeded with both within minutes. The basic design seems to be faulty: there is a very small difference between the steps of where the key positions the little locking pins, and quite high tolerances in manufacturing, which grow with use and wear, I guess. So a circular object that is moved around with some dexterity can easily fool the mechanism. Some high-end circular tumblers may be safe, but how could you tell? Buy something with an "extreme" key and the thief will walk on by your bike.
Best combination: U-lock + chain&padlock, or two different U-locks. Preferably with different types of lock tumblers, neither tubular.
Of course, bike value and neighborhood crime rate matters a lot.