Now that my hands hurt from playing Halo 3 for about three hours, let me give you some impressions.
First off, I watched the "making of" doc on the second disc (I didn't get the super expensive version with the helmet, just the special version). It's not actually a DVD, because they compressed the video to play it back in 720p through the console. Sweet. There is other stuff, but I haven't watched it yet. It was mostly to occupy me while eating dinner.
I've played through about two levels I think, choosing the "normal" mode instead of easy. The first thing you notice is just how beautiful the game is. They seem to have gotten out of the repeating hallway thing for the interior locations, and the outside stuff is just stunning. Lighting, textures, physics... it's all there. The dialog from non-players is so random, and seems to never repeat (good news if you got tired of "more enemy" in the first one). They even seem to have hidden goofy stuff. I went down an unexplored corridor to find a marine trying to get in a door, and the guy behind the door was like, "How do I know you don't have a brute with you threatening to kill you if you don't ask me to open the door?" It was actually a pretty funny exchange.
So far the only new weapon I've really used is the gravity hammer, which is ridiculous even though you can only use it at close range. You really do have to think about which weapons make the most sense in any given situation. It's hard too to find ways to duck and cover to avoid running into a field with 20 guys wanting to kill you. All good fun though.
I tried one deathmatch. With four players, 25 points, I got only 5. But hey, it was fun.
It'll get easier as players are more carefully ranked and better matched, so I'll have mostly other sucky players to be matched against.
Multi-player campaign mode is pretty neat even though I haven't tried it yet. I think I'm gonna try and convince Mike to do it when he gets his.
And of course, there's the story. I brushed up on the first two on Wikipedia, and they give a very clear recap in the manual for the game. Yes, I know how it ends (or allegedly ends), and that's cool. It's still neat to see it playing out.
Overall, I'm really impressed so far. As one of the biggest budget games expected to set a new record for first week sales, it really delivers thus far.