Several years back these were held in DC. In fact, back then, it seemed, they were all the rage. I'd attended them in DC, NYC, Boston, and Orlando. All were first rate and well run.
What works well for these types of events is security, organization, and discretion. A public place with a private room works well. Also, have one of our own controlling access. It takes a degree of patience, persistence, and diligence, but they can be great events for everyone, usually held on a weeknight.
Though I dropped out of this scene a few years back, I heard things started to fall apart for the DC M&Gs. One provider outed the details of a DC event on a public board. Then all sorts of vanilla folks showed up. And there were rumors of the owner of the hosting establishment video recording the comings and goings. For an NYC event (again, a few years back), the host was actually idiotic enough to invite a reporter to the gathering and the reporter did an expose. The host ended up banning himself from TER as they were basically a pariah at that point.
So, while I have many good memories of the M&G's here in DC (and elsewhere), I think there are a few ways that these things can go right and a few wildcards where things can become a disaster.