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I used this exact Gengar set in gen 5. It was one of my favorite sets ever. Like you said, you get so many free subs and switches while hitting almost everything for neutral or super effective. Awesome to see it still being used today.
I recently threw it on a gen 9 team that mainly utilizes...
I've been having fun against Annihilape with the old school sub/disable Gengar set. Of course you have to hope they aren't running normal terra type, but I usually see water.