I played the original with a friend of mine on the Xbox and enjoyed it. Nothing like beating the game on co-op.
We never got around to playing Halo 2 together, but he had it and like it. Since then I moved from Georgia back to Hawa'i.
I am now borrowing Halo 3 from a friend at work (my boss actually!).
Like the original Halo I'm expecting it to be a good game. I am not expecting a spiritual experience. I am not expecting to be so blown away I can talk about Halo and nothing else. I'm not expecting to be so impressed that I go out and buy the game for myself (indeed, there are other games i want but am putting off getting).
Halo never figured into my reasons to get an Xbox 360. Its just a game, and truth be told one in a genre that I'm not a huge fan of anymore.
So - any advice before I jump in? Won't be tonight - maybe during the week or (more likely) this weekend.
Oh, and at some point I really want to try Horde mode in Gears 2. Any advice there?