I haven't finished the campaign yet, i'm on the 6th level now, probably finish it tonight. It is a shame the campaign is rather short, I don't really feel like much has happened plot wise and I am almost 2/3's done! The level design is top-notch quality, every corner and room is filled with props and great textures. I don't have a favorite level yet, gonna wait to decide until after I finish. Even though the campaign is short, I can see a lot of replay in it by playing with friends and trying to find all the skulls (only found one so far).
Multiplayer is just as good as Halo 2, really, no need to change what isn't broken. I haven't jumped into this too much because I want to finish the campaign first but I did play a little forge with my friends online. That was interesting, I'm still learning the controllers and what the capabilities are. I don't have any real strategy with forge yet.
By far my favorite part of Halo 3 is the saved films. It blows my mind every time I watch one of either campaign or matchmaking. I love freezing time and rotating around characters, feels just like the Matrix. I only wish it could show me the best action at each moment instead of just having me fly around the whole time to see everything. What I mean is, if I am watching one of my multiplayer matches, I only know the whole fight from my perspective, there could have been many other great battles within that time that I didn't know about. It would take a ton of time for me to pause every second, fly to the next group in the heat of battle and watch them. I wish there was an option to tell it to "jump to the most action at the moment" in the match. That way I can just sit back and watch it like a movie and know I am seeing the best fights in the match.