I watched someone snap the lock off my Felt mountain bike 2 years ago, 3 days after I bought it.
I was working on the doors in the gay village in Manchester, I'd cycled to work and locked it up to a post on canal street. 1 hour into my shift a guy bent down at my bike, watched him take bolt cutters out and snap it clean off. Lucky really, as I didn't have my D lock, but my bike was 15 ft away.
I was waiting for him to get on before I would have gone over and knocked him off, luckily he walked to the canal and dropped the cutters over the side. Walking back to my bike, I put him on his behind and locked his arms and pinned him. Another lad from a club 2 doors down came and helped, and with it being a saturday night, we had police walking up and down regularly. 2 minutes later they nicked him.
It took 18 months to go through, but I receive £1 a month now for the next 30 months for the lock. Haha. :-/
My head would be to go all out if I saw it happen again to my own bike, but my heart says to leave it. Nowadays you never know who is carrying what.