At my local bike shop, the mechanics, for their own amusement, decided to try to beat the h*ll out of a couple of U-locks. One was a cheap $20 U-lock, and the other was a New York lock. With a tire jack, they were able to break and twist the $20 lock so that it looked like a piece of abstract sculpture. On the other hand, they spent about an hour trying to bust the New York lock. They were able to strip off some of the outer shell, but the main U-lock itself was unbroken. (I'm pretty sure they only tried normal mechanical means and did not use, say, expensive electrical power tools to try to destroy the lock. But since power tools need a power source, etc., they're nowhere near as common as other means that thieves would use to destroy bike locks.)

Of course, one thing with the New York lock: DON'T LOSE YOUR ONLY KEY. If you're down to one key, go get a spare made.