they are as useful as the guy who is using them. i only saw one fight where brass knuckles were used and that guy got his ass beat and his knuckles used against him. even with a weapon he still hit like a bitch and ended up getting jumped by the other guys friends
they are pretty effective I heard of people having to get staples in their head and their face was swollen like they had been in a car accident. if someone pulls brass knuckles on me im stabbing them immediately.