Well, I woke up this morning to a freshly downloaded Halo 3 Beta. I love it, but of course I expected that I would. I know you are all chomping at the bit to get my thoughts

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First and foremost, this is just feels like Halo, which is a good thing. It feels like an extension to Halo 2, but so far it seems worthy of a new game and not just an expansion pack. I've read discussions on whether this actually felt like Halo 3 or Halo 2.5 on a new console. Everything feels identical to Halo 2 in movement, pacing, and fighting.
Personally, I think the graphics look fantastic. I played my first match on Highground, and I think this definitely looks next gen. What's weird about Halo 3 is that at first glance, it looks just like Halo 2. But it's all the details that make it stand out. There are definitely more polygons on the screen, the textures look better, the lighting is WAY better, and the models look better. What more could you ask for? I've only played two matches thus far, with Snowbound being the second. Snowbound was a bit bland, but what can I expect with a bunch of snow?
I can't comment on weapons much, since I never changed from the default weapon. It was working fine. If you have a good aim, you can take someone down with one clip, which I like. Keep your aim true and make them miss a couple of times, and you'll have won the mini-battle (assuming they are using the same weapon). The grenades feel good, but they are what you'd expect. In my two matches, I could never hear any audial clues that you picked up the nades, and on my 52" HDTV, the grenade numbers in the upper-left are hard to just glance at, sometimes impossible on Snowbound by blending in with the sky. I just like to hear something to know that I nabbed them, as I also typically use that sound to know if I'm full of nades. If I'm fighting around nades, I can spam a few at my enemy whilst working my way to pick up more. I typically do this without ever taking my eyes off the center of the screen, especially when I'm in the middle of a fight.
I haven't played Valhalla yet, and I haven't played any team games, so not much vehicle use yet. I did hop on the new 4-wheeler, which was fast, but nothing special. Just another means to get around in, although more agile and quicker that all other vehicles.
One thing about the matchmaking system that I noticed different was how it automatically started finding another game for you as soon as you were done with the last one. Not that big of a deal, but in Halo 2 it would throw you and your party back to the party lobby, and you could view your stats and such from there. Now you have to quit to the lobby party, although the option is always there to view your stats while it's searching for another game. I like the new player profile screen. Shows what medals you got for the match, and explains them to you.
My stats:
http://www.bungie.net/Stats/PlayerStatsHalo3.aspx?player=blurredvision06
I did pretty damn well in my first two matches, placing 2nd and 1st respectively, with 44 kills and 26 deaths overall. I'm typically a middle-range skill player, so these stats are way too inflated. I also seemed to like melee'ing from behind, since I got 2 Assassin medals in each of my first two games.
Overall, I obviously like it. I'm looking forward to playing with you guys on some team games. Remember, my first two games aren't typically indicitive of my play, as I'm usually best suited as a support-type player. I die more than I kill.