Anybody who doubted 343 Industries, probably feels pretty stupid right now.
Campaign
It's been a long 5 years since Halo 3 but I've now played up to mission 4 of the Halo 4 campaign and honestly, it's pretty awesome. It takes place right after the events of Halo 3, but is an entirely new experience. I won't spoil anything but right off the beginning it is an absolute blast.
Multiplayer
Multiplayer is improved and is just as fun as I remember. Every map on the game is unique and well designed. I just can't complain about anything with the muliplayer, 343 has done a great job of bringing Bungie's fantastic multiplayer into Halo 4 and giving it a badly needed shot in the arm.
The Rest
Spartan Ops is a great replacement for Firefight and each week a new episode comes out so there's always something to do. Forge, as with the Forge in Halo Reach, has been updated to include more fun stuff and more ways to make maps to your liking. Custom Games is back again and is still an awesome place to test your skills and play your created maps with friends.
Overall the game has gone way over my expectations and has pleasantly surprised me in ways I thought only Bungie could. I'm glad to say that 343 Industries has done a perfect job of taking what Bungie started and turning it into something their own.