I support quickbanning Gengarite.
Firstly, the argument that "you can prepare for Mega Gengar" is total bull. You can't prepare for something you can't switch out of; it's just not possible. The only way to "prepare" for Mega Gengar is to have 6 Pokemon that can either kill it before it can move or that it can't trap, which is totally nonrealistic and extremely centralizing to a degree not even Ubers has seen. The problem is not beating Mega Gengar, the problem is beating it before it can do its job, which, given its insane stats, insane movepool that includes totally unfair support moves when comboed with Shadow Tag like DBond and Perish Song that the other trappers could only dream of, and excellent typing (which only got
better in Gen 6), is impossible.
Secondly, Mega Gengar isn't even
that frail. Attacks that would kill regular Gengar easily fail to OHKO Mega Gengar. Factoring in SR (Mega Gengar spinblocks btw), CB Scizor fails to OHKO with Bullet Punch, Ferrothorn fails to OHKO with Gyro Ball, even 252+ Atk Aegislash fails to OHKO with Shadow Sneak - that is STAB SE priority, for the record. On the whole, it takes hits much better than normal Gengar. Often it doesn't need to take a hit at all with useful Normal/Fighting immunities and an abundance of U-turn/Volt Switch users to give it a free pass into battle.
I think the reason why a lot of people in the general discussion thread in UT think its underwhelming is because they send it out and expect it to 6-0 teams or do something inane like that by itself. That is not what you use Mega Gengar for. It is not a bulldozer; rather, it is a sniper, used to eliminate one or two or even three high value targets on the enemy team to make way for your teammates. Being able to eliminate basically any target you want on the enemy team without them being able to do anything about it is
the ultimate support you can give any Pokemon. At worst, Mega Gengar is looking at a 1-to-1 trade, which is still in the user of Mega Gengar's advantage since the opponent only has ONE turn to react to Gengar before it can trap something - if it escapes your Ttar, which it can do easily with moves like Disable, Substitute, or even just Focus Blast (and if it has FBlast,
Tyranitar is the one getting trapped, not Gengar), you are screwed. And to those people who say Mega Gengar is weak, this
post presents pretty clearly the sheer amount of Pokemon Mega Gengar can trap or revenge with basically no effort. And that's not even factoring Destiny Bond at all or Perish Trap for anything but Blissey, which Gengar doesn't need to beat (Taunt), and Gastrodon. If you want to argue that we shouldn't quickban Mega Gengar because "Perish Trap doesn't exist!" in a metagame that will become obsolete in one month and for the temporary illegality of a move that is really only necessary to beat freaking Gastrodon of all things, I don't really know what to say to you.
tl;dr Mega Gengar is extraordinarily uncompetitive and should not be let anywhere near OU. It doesn't need a test - it needs the boot.