I've played maybe two hours total of this. The first bit, my TV was adjusted wrong for it, and I couldn't see much. I've corrected that problem and things are much better now.
It's a Halo game without Master Chief, which feels odd. Your character's run speed is pathetic, and way out of proportion with his ability to jump, which doesn't appear to have been altered from MC's levels. The new stamina system is interesting. Basically you have a tiny little shield that even one or two hits will deplete, and takes about ten seconds to regenerate. You also have health under the shield, and that health doesn't regenerate. You have to pick up medpacks to replenish it.
The art and level design are very nice - I'm enjoying the game as an art tour of New Mombasa quite a lot.
Co-op is enormously powerful in this game. The enemy AI isn't particularly smart, so hitting them from both sides is massively rewarded. Also, ammo is quite scarce, so the more people you have lugging along full guns, the better. I can see how stealth will be more important, too, due to the scarcity of ammo. (It takes a whole lot of bullets to put anything other than a grunt down...)
So far, the Covenant carbine is easily my favorite weapon. It does good damage, appears to be second only to the sniper rifles in accuracy, and the ammo capacity is pretty reasonable. I'm using it every chance I get, and rotating my other weapon slot with whatever else is handy. And you will rotate a lot, because as I said, ammo is quite scarce. I'm extremely bummed that there are no Sangheili (Elite) laser-swords in this game. Those were my favorite. I guess it makes sense, since the Sangheili have left the Covenant and allied with humanity by this point in the story.
I'm not sure I like the way they're telling the story. I realize that making the first level a night-time run through the first bit of the city is very dramatic. But I really think they would have been better served from a story clarity and character continuity perspective by showing a segment of Buck (Nathan Fillon, aka Mal from Firefly) going through the city during the day first. With his dialog and companions, he keeps the story going. And then have the lone, silent Rookie following a similar path, later at night. They do things vice-versa (Rookie night segments first) and I'm not enjoying it as much as I think I would the other way.