I became interested in BJJ in 94 or so after taking TKD and Aikido on and off for several years. I am a pretty big guy, 6' and around 260 or so. I went into the BJJ studio and went against their teacher, a brown belt under Nelson Monteiro, who was all of 170lbs on a good day.
We got on the mat and the guy says "Ok, now get me in a headlock or something." The guy then comes at me full speed, so I used a rolling foot-on-his-hip throw to slam him against the padded wall. Well, on the way down, he armbarred me that quick. I tapped right away. Throughout the sparring session I was armbarred probably 10 times and choked out once due to leg triangle. Very humbling experience.
Recently, I had shown my girlfriends younger brother some BJJ positions, strikes, etc. One night outside of my apartment, he was sucker punched by a gang member (right in front of me, out of the blue) that had about 60 or 70 pounds on him. Brother screamed, scuffled with the gang member for all of 30 seconds before he took him down into the full mount. Brother did left-right left-right punches for about twenty seconds before I had to pull him off the guy, whose face now looked like road kill.
Cops came, listened to what happened and let everyone go. Chalk up one for the good guys.