Genger definetely has decent HP, and you can simply use Life Drain and Shadow Dance to KO pokemon VERY easily if you get lucky while flipping. Then Gengar will be retreating it after those two attacks (even though it'll take two turns at least). That normal resistance is nice, protecting it from the normal powerhouses and some other Pokemon. Low retreat cost also helps, however since dark energy and dark pokemon became a bit more common since darkness energy became basic, it's quite costly playing it against a dark type monster with a decent attack.
Definetely useful if you have the evolution chain and something such as one of those Flygons, however it's a bit risky playing psychics up against dark types now, so I'd have to give it a 8.5/10.