Just some personal opinions:
Likes:
Campaign story seems to be shaping up well.
Incredible graphics.
4 player coop.
Great new multiplayer game types (VP, Territory etc.)
Dislikes:
Pistol still not the same as Halo1.
Multiplayer still feels more like Unreal Tournament.
Placement of weopens on multiplayer maps.
The fact that the second player has to be the Arbiter in coop.