Got it at midnight. Played through the campaign and Spartan Ops and I've been doing some multiplayer as well.
I've had a hell of a lot of fun with it. To start, the game is god damn beautiful, and gives Halo a much-needed graphical overhaul. The campaign was fun and probably had the best level design of any Halo game - each mission was enjoyable and had something that made it stand out from all the others (whereas, in pretty much every other Halo game, there's a mission/series of missions that are a drag/boring). The story was very enjoyable as well and works great on its own (while it is called the Reclaimer Trilogy, Halo 4 can stand entirely on its own). Things might be a bit confusing for those who are less familiar with the Halo universe's lore. For example (Halo 3 spoiler inbound)
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Spartan Ops is alright. It's some good shoot-'em-up fun with a fun cinematic to kick it all off. The story hasn't been anything special so far, but hey, there's still 9 more episodes left for this season and best of all they're all free, so I can' complain.
Multiplayer is great, although inevitably some people will dislike it. Personally, I haven't really enjoyed a Halo multiplayer since Halo 2. I wasn't sure if I'd like 4 due to all of the customization/load-out stuff, but 343i did a really good job. Grenades are kind of disappointing, though...it seems like they have an incredibly small effective radius.
Hardest thing to get used to are the new sound effects. 343i has built everything from the ground up, meaning that weapons and enemies sound entirely different than they did in past games. It's not a bad thing as the sound in Halo 4 is incredible, but it just takes awhile to get used to hearing such a drastically different sound effect when firing something like the plasma pistol (I've been hearing roughly the same sound for 11 years, change takes some time to get used to). Also, no more methane masks for grunts, so they don't sound crazy and high as a kite.
The musical score is okay. It's not bad, but it's certainly not amazing. Where Marty's music added to the games, the music in Halo 4 just accompanies the gameplay. It fits, but it rarely serves to make anything more intense/dramatic. There are one or two tracks that I really enjoyed, though.
Best gameplay moment? The start of the eighth (final) level. Holy shit. Probably the coolest thing I've ever done in a Halo game and an awesome nod to some other sci-fi stuff. I won't bother going into detail because I don't want to spoil anything, but 343i pays homage to some other sci-fi in an awesome way.