Translated by Ralph Anzarouth and an anonymous friend
Flattery is the exact opposite of rebuke. It consists in praising and glorifying somebody with sweet talk by agreeing with what he says or applauding his deeds, even though they are wrong and improper, in order to receive benefit from that person, just like fishing for a whale with a rod1.
However, if one speaks pleasantly in order to be agreeable to his fellow men and to find favor in their eyes with no intention of receiving any benefit from that or for any ulterior motive, this is called wisdom, since it is derived from the moral teaching of our Sages of blessed memory, who said that one should always be on good terms with his fellow men.
The bunch of flatterers resemble the siren (mermaid), which is a sea creature, whose upper half takes the form of young woman and its lower half takes the form of a fish: it dwells among the rocks and dangerous places at sea. When a ship passes by the siren begins to sing in such a sweet tone that it causes the crew of the ship, sailors and officers, to fall asleep, after which it comes on to the ship, it kills and it eats those on board.
King Solomon said (Proverbs 11, 9): "The flatterer will destroy his fellow man with his mouth and the righteous will be saved by wisdom". Elihu said (Job 36, 13): "Evil flatterers will be angry and will not plead when they are in chains". And Job said (Job 27, 8): "What hope has the flatterer from fraud, since the Lord will take away his soul?". In Talmud Bavli, tractate Sotah (folio 41b) Rabbi Elazar said that every man who is tainted with the trait of flattery is cursed even by fetuses who are still in their mothers' womb, as it is said (Proverbs 24, 24): "The one who says to an evil person 'you are righteous', the peoples will curse him and the nations will execrate him". And further sayings of Rabbi Elazar2 are brought down there which greatly despise this trait. Even more than this, it is related there (Talmud Bavli, tractate Sotah 41a) that king Agrippas stood and read3, and when he reached the sentence (Deut. 17, 15) "You will not be allowed to appoint over you a stranger who is not your brother", tears poured from his eyes. The people said to him: "Do not fear, King Agrippas, you are our brother4 etc.".
And in truth the punishment for the sin of the flatterers is more than man can bear, because they cause the people to sin and for this they will stand judgement.
Tullius said:"Treat all your fellow men pleasantly, but do not flatter and do not make too many friends". Ovid said: "Beneath the honeyed words of the flatterer a bitter root is concealed". Aesop said: "Pleasant speeches cause evil deeds".
Even the dog likes the bone as long as meat is found on it and the bee will like the flower as long it retains its beauty.
Cato said: "When somebody praises you, remember to retain your judgement; do not believe other people's opinion more than your own".
Plato said: "Don't trust someone who praises you excessively, since he will also abuse you more than you deserve, just as the scorpion kills with its mouth and stings with its tail".
Socrates said: "In the same way as the grass covers the earth, the flatterer will cover faults".
Concerning flattery, Aesop in his fables told about the fox which saw a raven with a piece of cheese in its mouth and tried to devise a way to take the cheese. It began to praise and exalt the raven and said: "My brother! There does not exists among all the fowls and birds of the air one more attractive and beautiful than you. If the sound of your singing is pleasant and enjoyable to same extent as the the beauty of your stunning appearance, there would be none to compare or compete with you in all the universe". On hearing its praises, the raven wished to show off its singing voice and opened its mouth to sing. The cheese fell and the fox took it and said: "The praises are for you and the cheese is for me". In this way, the raven was left ashamed and humiliated.
Notes of the translators:
 Quote from Job 40, 25.
 The text says Rabbi Eliezer, but current Gemara says Rabbi Elazar.
 This was the traditional kings' scriptural reading of Parashat Hakhel, as required by the text of Deut. 31, 10-13.
 According to the rules, king Agrippas was not eligible for kingship. Therefore the Sages criticized the flatterers for their reply.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 17 about flattery is found at pages 36-38.