Although towards the beginning of a suspect I was of the opinion that Mega Mence was broken due to a combination of its sheer power, ability, effectiveness when paired w/ certain redirectors (Jirachi), etc... my opinion on it has shifted to not broken.
Despite a lot of mence c-teaming on the current ladder, I still decided to make use of a team with bulky DD mence, follow me jirachi, and some other support such as tailwind on the ladder to get a better feel for its capabilities. In my experience, while a mence with a DD safely under its belt is indeed frightening, even moreso when behind a substitute, the problem is that it usually takes a fair bit of support from its partner to get mence set up in this fashion. Its bulk coupled with intimidate certainly increases its setup capabilities and potential, but its still prone to chip damage while it sets up, namely fake out, weather damage, substitute, spread moves such as rock slide, icy wind, hyper voice, etc... Another shortcoming with mence is that it can struggle versus certain playstyles/support options, namely tailwind, rain, and trick room. I acknowledge that mence's checks can and do vary depending on the set (mence w/out sub is more vulnerable to status and intimidate, mence w/out dd has trouble breaking fatter threats such as Rotom-W, Lando-T, Rotom-H, etc... Mence with more coverage essentially lets it bypass certain checks at the significant cost of being checked easier by other mons and support options such as status, fake out, and redirection [argument goes both ways])... the main point is that mence's checks do vary, but aren't insanely obscure (sure there do exist some less than viable options to bypass it, but there are certainly viable options that can be slid into a team), and mence can be checked by tactfully applying pressure on it with WoW, t-wave, fake out, redirection, etc.
So these are pretty much my thoughts on mega mence. Is it powerful? Bulky? A metagame titan? Sure, but accounting for its presence through teambuilder should allow teams to apply adequate pressure versus mence so that its not an overpowering threat (at least, this was my experience when building for and against it). Either way, if it does become apparent later that Mega Mence causes extreme unbalance to the metagame, we could definitely take a second look at it. However, I don't believe this right now, so I'm going no ban.