Multiplayer:
I have had really long wait times for matchmaking, but I've gotten into a couple of games. Joining people takes patience and sometimes requires you to quit the game and restart. We also learned at Customs Night that you cannot join a game that's already in progress.

So if your buddy is playing Zanzibar solo and running around, killing time, you have to wait for him/her to return to the lobby before you can join up, even if it's "Friends Only."

Controls
I think I've pegged why my control scheme randomly changes, without changing how it's listed in the Settings Tab. Starting a new round in Forge seems to cause this bug, but I don't think it happens in Multiplayer Custom Games. In either case, you just open the settings tab and click "A" to re-select your settings.

Returning to Lobby
When we finished a game of Customs last night, we usually returned to our own separate lobbies instead of one together. We could usually just rejoin, so more of an extra step than a major bug. Occasionally, the game would give us a "Join Failed" bug, but waiting a few minutes or rebooting Halo seemed to fix it.

Custom Game Lag
Not a whole lot of it, actually. Out of all our games last evening, we only really had two laggy moments and they lasted for less than 30-seconds each.

Overall, I am having a lot of fun with MCC and I know 343 Industries is working very hard to get everything working for us. I think people are overreacting for the most part, but that's not to say it isn't frustrating.

Hope that helps. :)