Poison does 1.2× DMG on the match where the ability activates, and for all matches thereafter, Poison-type Pokémon will deal 1.5× DMG.
Poison lasts for seven turns after activating, as opposed to the usual three turns for things like Burn, Sleep, etc. This was tested on Ampharos after noticing that it wasn't going away after three turns while levelling up SS Gulpin on Victini.
As for what is immune to it, I haven't tested that yet, but Psychic/Electric aren't immune and I hope Grass and Fairy are not immune either.
So yeah, Poison is actually pretty good for a status affliction.
EDIT: Can confirm, Poison does work on Fairy-types. Tested on Gardevoir.
From Its_A_Random on Reddit
From Micloti on Reddit
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