Have you played Halo 2? Well, then you pretty much have already played Halo 3.
The multiplayer is fun and keeps you coming back for more, that is if you like shooting your opponent 60 times before he dies. I'm not knocking the formula because it is a good one. The balance of tossing grenades, getting up close and personal and smacking your enemy in the head, or sniping from a distance is pretty sweet and takes some skill. I am aware that you can alter maps and change the amount of damage you can take in custom games, but I just don't care to spend my time altering the game or tying to get people to play my custom games with me.
I gave this game a 7 because it doesn't really add that much new content. It's the same old thing, but it's still a great game. So if you haven't played Halo games before then you will probably love it. But, if your tired of the same old thing and think your enemy should fall to the ground if you shoot them in the face one time, then Modern Warfare 2 is what you need to get. And after playing MW2, Halo just seems sluggish and ugly.