Still hardly working. Occasionally gotten into a game, and every game has either had lopsided teams, got booted during loading, or was super laggy (wasn't this game supposed to have dedicated servers?)
Also, after just completing a game, there was no option to leave the end-game scoreboard. I had to go to Xbox Home and force Halo: MCC to quit. Upon restarting the first time, I received Xbox error messages about being unable to sync live data, and Halo would give me "Downloading Latest Data" for a long time. Eventually I got back in, but just now it's happened again and I'm just sitting here staring at "Downloading Latest Data: Downloading the latest matchmaking data. Please wait." forever.
Really, though. The post-game score screen missing the "back" function is a pretty impressive glitch.
EDIT: That's what I get for not reading through the previous replies. They probably are rebooting the servers.