Ok, played it for about 5 hours in co-op last night, and an hour this morning before work, I'll weigh in on the game so far.
Graphics
Certainly a mixed bag here. The lighting, foliage and texture detail are all very nice, and the backgrounds in some areas are truly gorgeous. Later in the game, around level 5, it really does feel like it rivals Gears in quality, the landscapes are breathtaking.
But then, things like the aliasing, the H2 quality animations, the hard to see weapons and the mountain/ground textures severely detract from the overall image quality.
So, it has moments where it looks truly unimpressive, and others where it's one of the most beautiful games on the platform.
Some good aliasing, and overhauled animation engine sure would've made a world of different though.
Gameplay
Difficulty/AI
Ok, is it just me or is this game significantly easier than Halo 2?? The AI is not improved as far as I can tell, and the brutes are nowhere near as aggressive as the Elite's where in H2, so overall it's a much less challenging experience. On the flip side, the Snipers are extremely accurate, almost too much so.
Co-op on legendary (2players) was quite tough, but mainly because the enemies had a TON of hit points. Playing by myself on Heroic (only the 1st level so far), it's been quite easy. It's still rewarding, but is lacking a little bit of that adrenaline pumping feel that Halo 2 had. In other words, it lets you take your time alot more.
Level Design
THis is where the game excels, the levels are truly the cream of the crop in FPS titles, and the outdoor levels offer great opportunity to experiment with tactics, if you make use of the open environments.
Initially I was spending alot of times sitting in one spot, sniping enemies from afar with the battle rifle, but when I decided to bring the combat in closer, and began to ustilize the many vantage points and alternate routes, the combat really came into it's own.
Audio
So far a big dissapointment personally. Certainly does not capture the epic feel of Halo 1, and the levels seems very odd. Gunfire, and music are quite soft&quiet, while enemy voices are very loud.
I would've really enjoyed an option to turn up the background music and gunfire, as it stands the music is simply lost alot of the time, and doesn't stand out enough during the battles.
VS Gears of War
In a gameplay comparison to gears, I feel Halo wins easily. The 'stop and pop' gameplay in Gears is nowhere near as difficult, heart pumping or as fast paced as it is in Halo. Even on Insane gears consisted of little more than popping out and shooting guys in the face, and you rarely found yourself in danger.
With Halo you can play the levels in a variety of ways, up close, or from a distance, and you will have many instances where you are on the brink of death but manage to survive.
The AI in the two games are evenly matched, I don't say this because gears had great AI, but because Halo 3's has not improved much, if at all, over Halo 2. The enemies in Gears are certainly more aggressive, and flank you much more often, so they win in that regard. But they also lack the intelligence to regroup, to flee, and to hide which we've always seen in Halo. I give this one a slight edge to Halo, although it should've beena blowout, had they expanded on Halo 2 as I expected.
All in all, a very fun game, I would give it in the range of a 9/10. There's not much to pull it down, questionable GFX at some points, uninspiring audio and unimpressive AI are the main gripes I have.