Is rebuke the best thing to hear from a Chacham?
Ibn Ezra: All the more so, it is good to hear his words of Chachmah. The verse teaches that even the rebuke of a Chacham who became angry, it helps and delights the Neshamah more than hearing song of Kesilim.
Why is it better to hear a Chacham's rebuke than song of Kesilim?
Rashbam: The former makes him wiser; he does not understand anything from the latter, he just hears folly.
R. Avigdor: Chachamim expound and scare people about the punishments that will come for their sins. Shir Kesilim is of translators {who raise their voices to make the nation hear 1 - Koheles Rabah}.
Ri Kara #1: A Chacham's rebuke saddens [the recipient] immediately. Even so, it is better than song of Kesilim, which gladdens the heart immediately, for one who takes rebuke, he takes Musar and it brings him to Simchah. One who turns his heart to hear song of Kesilim, he neglects his work, and this brings him to poverty.
Ri Kara #2: It is good to go to hear a Chacham's rebuke, for he becomes wise and retracts from his folly. One who hears the song of Kesilim, he despairs from (neglects) all his needs, for his heart is drawn after it.
Seforno: Even though rebuke saddens him, he attains from it fixing the crooked. The song of Kesilim makes him rejoice temporarily [but there is no lasting benefit from it].
Metzudas David: A Chacham rebukes only in order to straighten a person's ways. This benefits the hearer, even though it scares his heart.
Maharzu (on the Midrash): They say the rebuke like a song, and ruin the intent. Radal (on the Midrash): The Ramban's text is 'Chazanim' (and not 'translators'). They add Piyutim to the fixed Tefilah.
Why does it say "me'Ish"?
Metzudas David: This is even if he is an esteemed person who fears Elokim, that he will become light-headed due to hearing the Shir. There is benefit from the Shir - it gladdens the heart and removes worry. Even so, since Kesilim sing, inevitably he will hear light-headedness, and such a thought will enter his heart temporarily.
