So I ended up getting this as something to tide me over until Black Ops 2 next week. Overall the game is pretty fun. I played through the first mission, and other than the fact that it seemed like I spent half my time needing ammo, it was awesome.
As for the multiplayer, it's okay. There's some cool new weapons and vehicles. The Mantis is an unstoppable force if you can get one. My two complaints are... 1) Why the hell is the DMR back as the primary slayer rifle? The BR is way better. 2) The maps are entirely too big. The days of close quarters slayer games on maps like Guardian, Lockout, or Foundation are a thing of the past. A map like Zanzibar/Last Resort from Halo 2/3 would be one of the smaller maps on Halo 4.
Also apparently a thing of the past are the days where you could get a 4v4 game almost every time without somebody quitting. Probably 75% of the games I've played so far had at least one, and usually multiple, people quit. Oftentimes it was before the game even launched. Hopefully this is just bad luck and not par for the course with this game.