So, I received the review version and have been playing 4:38 hours and I completed it for 57%. I'm playing on Hard. That's the only difficulty you should be going for (and I'm not one of those 'if you haven't played Halo on Legendary in co-op you should shut up about it'-guys). This is because all the combat mechanics will be needed and the game really, really shines that way.
The combat is incredibly good. All the different elements (taking cover, switching cover, blind-firing, melee, shoot from hanging positions, grenades) are so extremely well combined and you need them ALL to succeed. It plays very, very fluently and just everything feels right.
Platforming is also a lot of fun and the locations are great. Characters are very believable, with very good voice-acting and very good motion-capturing. You really care about them during the storyline.
So, no negative points yet. I'll keep you posted and feel free to ask anything. But don't know if I got enough time to post here.