It's not the best use of stardust, but it's strong enough as an attacker to justify using stardust on it and it's unique (not everyone has a good Gengar, let alone a good legacy moveset Gengar). If it'll allow you to enjoy the game more and if you don't have better options to power up, then enjoy your Gengar and max it out.
GamePress
Gengar SC/SB
Gengar looks so freaking cool. I'm level 33 have 98% best moves at cp2000 looking for a reason to max him out. Honestly though have I have no idea what I would use him to attach into and seems like a waste of dust. Any advice for those that love him what do you use him against or someone tell me it's a complete waste.