These were pretty big when I lived in Denver in the mid-2000s, mostly among the bike messengers. I never did one - those messengers were fast, fearless, drafted anything and everything, and really knew how to literally cut through alleys to avoid having to stop. I rode around town with messengers a few times, and it was pretty wild!
I've heard of a few events locally (in La Crosse, WI) but they seem to be less like races and more like moderately paced urban rides for riders somewhere in between full-kit wearing roadies and the slow bicycle or pub crawl riders.