Rav Sadya Gaon: There is a common Hevel that occurs in this world. Metzudas David - "Al ha'Aretz" refers to this world.

Rashi: It is something that is Mavhil (confounds) people.

Rid: It is a great evil.

R. Avigdor citing Horiyos 10b #1: Tzadikim are fortunate. They are punished (to cleanse them from their sins) in this world (where the punishments are much lighter) like is proper for Resha'im in the world to come. Woe to Resha'im! They receive all the reward for their Mitzvos in this world, whereas Tzadikim get (infinitely greater) reward in the world to come ¹ .

Horiyos 10b #2: Tzadikim are fortunate! They get reward (in addition to reward in the world to come) in this world, like Resha'im get (all their reward) in this world. Woe to Resha'im, they are punished (also) in this world, like Tzadikim get (all their) punishments in this world.

Rav Sadya Gaon: This is occurs to a [small] number of Tzadikim and Resha'im.

Rashi: The Tzadikim get evil, like Resha'im's deeds, and the Resha'im get good, like Tzadikim's deeds.

Rid: Not only are Resha'im not punished in this world - even more, Tzadikim get like is proper for Resha'im [and Resha'im get like is proper for Tzadikim]!

Seforno: There are Tzadikim who get a bad reputation like Resha'im, e.g. a foolish Chasid. And Resha'im get like is proper for Tzadikim, e.g. a crafty Rasha (he appears to do virtuous acts) and blows of Perushim (e.g. they bump into walls, like one who closes his eyes to avoid looking at women - Sotah 22b).

Ibn Ezra: Since Tzadikim can get like is proper for Resha'im and vice-versa, the difference between them is Hevel. This is like "va'Ani Kim'at Natayu Raglai