A famous bearer of this name is Sammy Clay (originally Sammy Klayman), who is one of the two lead Jewish boys in the book The Amazing Adventures of Kavalier and Clay by Michael Chabon, who writes a famous comic book with his cousin during World War II.
I am not a big fan of the names Samuel or Sam, yet I absolutely love the name Sammy for a boy. I think it can work for both little boys and adults, as long as they aren't too serious!