The first couple of missions have been the best so far (although I'm only half way through mission 6 at the moment). Pretty impressed, the graphics are immense and whilst it sounds very tinny, the soundtrack is very good.
Not too sure what I make of some of the new weapons, they lack punch and seem underpowered. The Promethean Bolt Shot is probably one of the most pointless guns I've used in a game.
I won't give too much away, but the story is incredible. The Intro is very impressive and there's a cutscene at the end of Mission 5 where you meet someone who's eluded to in the books and previous game, that was jaw dropping (not graphically).
Overall, I'm really enjoying so far and I've not even started Multiplayer or Spartan Ops yet.