Great campaign, Amazing multiplayer.
Then there is multiplayer. This is the one of the best multiplayer experiences you can find anywhere right now. It is really fun and engaging. It is even more fun when you play with your friends. There are several different modes that you will no doubt find a mode that you will enjoy. The community surrounding Halo is huge. You can go on to bungie.net and view your career stats. Custum games is where you can custimize your matches and invite your friends (or whoever) to play a specific game variant, such as slayer, infection, juggernaught or whatever. You can custimize the game details in so many ways the possibilities are endless. You can really get creative and create interesting modes to play with.
Forge mode is where you can edit item placement throughout the entire level. You can't mess with the level geography, only the items. This might sound boring, but this is one of the most amazing features this game has to offer. For awhile you might just make piles of fusion coils and blow yourself up, but eventually you will get creative and you can make really cool creations.
Another cool feature is save films. This is where you can go and view your previous forge, multiplayer, or campaign sessions. There is a list of 25 or your most recent games, but from that list you can save as many as you would like. While you are viewing you previous battles you can take screenshots or film short clips.
Then when you make a cool map through forge, or an interesting game variant, save film/clip, or screenshot you can save it to your file share. Now your file share is a place where you post your favorite material. Then any player that you play a match with can view your file share and download the material on it. Naturally, you can download stuff of off other people's file shares.
Halo 3 has so many different things that come together so well, it makes it one of the best game on the market today.