I got one of those things at a state fair when I was a little kid and set about immediately cutting myself trying to flip it open and look cool. My mom took it from me shortly thereafter. That ended the short saga of WoahWoah and the butterfly knife.
At this point, I can’t think of any reason I would want one. People that can use them well look pretty cool, but as a purposeful knife, they seem pointlessly complicated and prone to user error.