As a mostly PC gamer I have to say I am very impressed with this game. Very enjoyable.
The campaign was really well done. I enjoyed it from start to finish. The production qualities and polish were truly outstanding, but that is to be expected for a Halo game. What I didn't expect (and didn't really believe when people said it) was the that story would be so good. I was really impressed with the story, it was very entertaining and flowed really well, it kept me hooked throughout the whole campaign. Save for a few confusing parts where I didn't get the references (I am not really familiar with the lore) I was thoroughly impressed. The voice acting and facial animations were outstanding as well. If this is the start of a new trilogy, you can consider me hooked for the next two.
Combat was smooth as ever and the new enemies were fun to fight. You still fight a lot of covenant, but the new enemies are really cool IMO. The knights and crawlers were pretty interesting since they are highly mobile and like to flank you. I'm not all that good (read: pretty terrible) with the controller, but I didn't have to much trouble on normal and am going to give heroic a try. There were a few annoying parts in terms of running out of ammo and the flying drone were a really PITA for me since they evaded once you damaged them, but overall I enjoyed the combat. The mission variety was also quite good. Vehicle sequences, outdoor area, tight cooridors, even flying missions (I particularly liked flying the Broadsword).
Weapon variety was a bit lacking in some regards. I miss having a weapon with near infinite ammo like the pulse rifle of previous games. It made long stretches more manageable, in this game I occasionally found myself backtracking to scrounge ammo. But overall the new weapons were pretty cool, especially the Binary Rifle and Incendiary Cannon.
As for multiplayer it is pretty fun. I'm not very good with the controller when it comes to shooters, but I am able to manage reasonably well. The available maps are pretty good, but could use more variety since everyone seems to vote the same maps over and over. Lots of variety and customization available for game types, with fun ways to mix up options and item/vehicle availability. I personally really enjoyed Regicide, a free-for-all and the top player is hunted by everyone else giving more points for killing the top player. Gets pretty chaotic, but very enjoyable. The new Forerunner weapons don't really seem all the useful in MP to me. The Suppressor isn't as good as the standard AR, and the Light Rifle is basically on par with the DMR. Decent variety of unlocks in terms of perks and armor abilities. It takes about 8 hours or so to unlock all the upgrades and weapons, and everything after lvl22 is cosmetic. Each level up rewards you with a skill point and it is just a matter of buying what you want in terms of new perks/weapons. Overall I am enjoying it, will hopefully have some opportunities to take my 360 over to my friend's house and play over the holidays, which should be fun as well.
Normally I break down all my reviews into pros/cons, but having not played many shooters on the 360 or Halo specifically I'll just leave it to a a text review since my opinions are fairly subjective. IMO it was well worth the money and I'll continue playing for the higher difficulty SP runs and plenty of multiplayer. If you are even remotely interested you should pick this game up, all the good things people are saying about Halo 4 are true from my experience. Great game, 343 really managed to pick up where Bungie left off and improve just about everything.