Gengar was hyped when SF spoilers came out, then it didn't really appear at Cities except in clunky fail builds with Dusknoir.
Then it got rediscovered and paired with Crobat G and Cresselia, and became a top tier deck for States.
Then people got wise about playing around Fainting Spell, and putting Unown G back in their decks.
Then it found new friends in Machamp and Nidoqueen (not in the same deck, please), and started causing problems again.
Gengar has had a really up-and-down history in this format, but the fact is that its nasty Power and cheap snipe attack can spell trouble for ANY deck. It isn't BDIF or anything, but it is ALWAYS a threat.
8/10