The Joker knife is great.  It is a single action (means it pops out with the auto button but to reset you must push the button and pull the blade back into the cocked position) with some real speed behind it.  You push the button and the blade comes out fast on this auto knife.
The blade is double sided (sharp on both sides).  The best part of this knife is it looks exactly like the one the Joker uses in the last Batman movie.  A fun auto knife.  Great for a collection, as a novelty, or for just all around kicks.