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Not even slightly. As of Gen 5 at least. Unless you're just using it for in-game or something.
90% of the time, your opponents will be killing you before you can use it if they have enough stat boosts that you're even considering using the move. From an offensive end, Poison probably one of the worst of Gengar's offensive options, being resisted and immune by so many Pokemon while not having much extra coverage that Ghost doesn't already have. So if you wanted to run any other moves, like Disable and Substitute or something, you'd be even more fucked than usual by any steel types, a standard drapion, etc.
Overall it's just a bad idea. Ask again in Gen 6 when fairies try to take over the metagame and then when they're shit fairy users electivire syndrome the shit out of them to force them into use anyway.
