What do other characters say about Hamlet?

Polonius believes Hamlet to be completely mad.

He knew me not at first; he said I

was a fishmonger: he is far gone, far gone: and

truly in my youth I suffered much extremity for

love; very near this.

Act II/ Scene II

Polonius thinks he is mad for Ophelia's love. Claudius believes that his madness has some method and indeed he may not be mad. Laertes thinks him a malicious killer. About himself, he compares himself to a recorder which his friends are trying to play.