I have a polydactyl cat where there isn't really room for the extra toe and might be a little uncomfortable. She can't stand anybody messing with her feet, we are told she's very spooky because children kept trying to touch her paws. I can pick her up and easily hold her upside down like a baby at the food bowl if we're alone but if somebody else is around she's very hard to handle.

I had a friend in college who had a similar condition, I knew him for a month before one day I looked at his hand and I said "Dude, you've got six fingers!" In his case the sixth finger was kinda a cross between a pinkie and thumb and not very useful.