Koffing is one of those great commons. I personally won a game with 2
Koffings, only losing one Pokemon (It was against a deck with grass
weakness). The reason he's so good is that he does damage (not much),
and can cause more damage whether the defending Pokemon is poisoned or
confused. Poison is the best bet, since you can knock out a Pokemon
pretty fast with it. And that tails flipped from a confused Pokemon
could mark the turning point of the game. He's not that weak and can
withstand most attacks (not Blastoise, Charizard, Venusaur, etc.) with
50HP. All in all, Koffing is a great card to have on your team.

Arcanine654