I just finished the game on insanely difficult nightmare mode; it was okay, not great.
The plot is just a mess, from Master Chief taking off his helmet to a cross-over with Mass Effect
The ending was well done though; they split it up among several difficulties and you have to piece it together, although my favorite boss was the giant chicken robot which you fight with laser swords.
Also open-world mode, THE BEST; seriously I was playing and was like, I wish this had some skyrim in it; what do you know; It happened. Though the vampires were a bit much.