It's good but not game-winning because it's a 1-mana weapon and Blizzard obviously tested this already. With Deadly Poison, it's a 3/3 weapon for 1-mana which is only ok in the late-game. With Envenom Weapon, it can destroy 3 minions but Rogue's health is a problem. 1-attack means Leeching Poison is useless, unless you're willing to play a weapon Rogue that revolves around a 1-mana legendary weapon.
In conclusion, heavily overrated much like Grumble, Worldshaker.
Just saying, the meta right now really relies on high rolls and this is another one adding to it, with no big risk of running the card like Kelesseth is.