Can only remember one incident when I was younger involving a U-Lock...
I was a teenager in the late 90s and I had a brand new bike - a Giant Boulder in two-tone silver/red. (I still miss that bike 20 years later) I was riding down a marked cycle lane on the seafront promenade, it was early Sunday morning and virtually deserted - except for one crazy guy who ran over and tried to hit me with a wine bottle as I passed by him.
I immediately assumed it was an attempted mugging or something - so I yanked on the brakes and skidded round 180, leaning the bike on the railings and grabbing my U-lock out my backpack - a big hefty cast iron thing with the 'U' wrapped in bar tape so as not to scratch my frame - which made a convenient grippy handle as well.
The guy who had swung at me looked shocked as he saw it, and started walking backwards away from me as I got closer. I broke into a run with the lock raised over my head and as he turned to try and run, I caught him square in his back and sent him flat on his face, then jumped on his back whilst he was down.
I remember thinking to myself "If I aim for his head, that's attempted murder." - so I pinned his arm down with my knee and smashed the lock repeatedly into the back of his hand and fingers, and once into his wrist for good measure. He started screaming in pain so I figured I had at least broken a couple of his fingers. I got up off him and walked away, I issued a couple death threats as I left. I was worried about getting arrested after that, but the cops never came for me.