I believe, from what I have seen and read in this thread, that Mega-Gengar may indeed be broken if played properly.
However, I do not believe that it warrants a quick ban. My reasoning for this is fairly straightforward. Blaziken was quick-banned because it had a clear negative and centralizing effect on the metagame as it stood. The detrimental effect was clear, and in obvious play. While Mega-Gengar may indeed have the potential to be a massively broken, or at the very least, incredibly centralizing portion of the metagame, nothing I have seen so far indicates that it is this way right now.
The metagame is currently in extreme flux. Further, that state is going to remain throughout the entire duration left before the release of Pokébank. Playstyles are massively shifting, the new mechanics are still being adjusted to, and we are going to have a whole influx of new Pokémon and new changes to adapt to in a moment.
If Smogon is going to institute a quick-ban, I think that the Blaziken ban is the only way I should do it. If, at any point, Mega-Gengar is demonstrating a clear and negative effect on the metagame, ban it then, and ban it freely. Until and unless it reaches such a point, however, a ban is premature.
I honestly don't think we should get any more bans to the current, non-Pokébank OU, as it is simply not going to survive as a playable tier long enough to warrant a ban, especially as we have no indication that the non-Pokébank OU is currently being inundated with Mega-Gengars that are punching holes in every team and setting up sweepers for free.
Is Mega-Gengar broken? Quite probably. Does that mean it should be banned? Yes. Does that mean it should be banned before it proves that it is broken or demonstrates a negative effect on the metagame? I don't believe so.
Although I admit that I'm not familiar enough with how the metagame forms to know how critical pulling such a threat during the formation of the metagame and strategies may be. If it is required to get Mega-Gengar out of there early, in order to prevent it from having an centralizing or teambuilding effect that will likely persist long after it has been removed, then I can see the point of quick-banning it. Otherwise, I think that waiting for testing and results is the more prudent and healthy decision.