"Be prepared to give a negative review. If you give one to the work of a friend, and they're not your friend any more, they weren't ever your friend. As Robert Altman once told me, 'If you never gave me a bad review, what would a good review mean?' He was a great man. He thought over what he had said, and added: 'But all your bad reviews of my films have been wrong.'"
The late Roger Ebert offers some typically sage advice in his guide to being a critic.
If it's Ebert at his most caustic you're after, then look no further.
(Thanks to Megan and Faith for the links.)