Wow! Now any attack/ability/effect that also deals poison will deal one extra counter of damage, too! This is generally quite a versatile card, as many of the small, opening attacks that the "Poison" {P} Pokemon have include Poison. There are even attacks in this format that cause a "toxic" poison, by putting 2 damage counters instead of 1. Combine that with Virbank Gym, and they're hit with a whopping 40 damage from poison alone. That'll really rack up easy KO's if you can lock an opponent out of using Switch via Trainer Lock, or trap a Pokemon to be stuck on active with no way to retreat via Catcher or Paralysis.
!! Speak of the devil, we have a popular deck that can do that in the form of Accelgor/Gothitelle/Mew EX! I'm VERY confident we will gain this card in Boundaries Crossed, right? So this is all an exciting prospect that's well on its way!
8/10 for playability: It limits its oomph to effects that do Poison, but cards that show off one aspect of the game, like certain types of Pokemon, always adds a stylish flair, as well as bringing competitive power!!