My initial impressions.
1. This is the game I have been waiting for since Halo 4. By far the best Halo and best FPS I have played so far.
2. Graphics are INSANE. Very close to the best I've ever seen on any console, I honestly didn't think the 360 was capable of this amount of graphical and technical detail. Environments are huge and the draw distance is breathtaking. Also the facial animations are the best I have ever seen.
3. Multiplayer is the best I have ever played by far. Even when I get owned I am having fun. Everything blends together so perfectly and it takes the best of COD and the best of Halo to form the most excellent multiplayer I've ever played. Only gripes are the AR and SAW are a bit overpowered (especially the SAW).
4. I'm only on the first mission in campaign but so far it's also the best campaign I've ever played. Shows a human side of Master Chief and Cortana that I never thought I would see. Perfect blend of action, cinematic, and excellently crafted story.
I haven't played Spartan Ops or Forge or messed around with theater.
So overall impressions is that this game is AMAZING! Game of the year and of all time for me.
Although I was watching for numbers of people online and it looks like it is less than Halo: Reach's first day (the maximum amount of people I saw was about less than 250k people online playing MP) :/ I hope the word of mouth really carries the game.