Mr. Saturn has been a notable item since Melee, but I've never liked him much. He originates from the game Earthbound (Ness' home game) but I've never really played the game so I can't tell you much about him there.
In Smash, he's a walking item that can be picked up and thrown.
He can shatter shields quite easily. Which would be useful if opponents wouldn't see it coming and rather dodge or take the petty damage it inflicts.
Typically if I have the choice, I turn Mr. Saturn in the off selection when customizing a multiplayer match.