The Campaign is fun and the Multiplayer keeps you coming back; and the Forge mode, well just read my full review.
The gameplay still revolves around the basic trio of your guns, your grenades, and your good ole' melee attack. While this formula is still tried and true, several weapons have been added. The Assault Rifle is back in action and all the old favorites are back too. The Battle Rifle returns from Halo 2, and it comes across as a more accurate long-range weapon this time. Some additions to your arsenal include the Brute Spiker, a dual-wieldable rapid-firing automatic weapon that spits out thick, red-hot needles. Another Brute weapon is the Mauler, a daul-wieldable one-handed shotgun which can basically level opponents up close. The final big addition (besides the Spartan Laser which is a bad-ass anti-vehicle laser cannon) is the ability to rip turrets from the ground to use them in combat. The human gatling guns, the Covenent plasma cannons and a human flamethrower all serve as powerful support weapons. This added power comes at a price though, your movement speed is slowed down when wielding these weapons. You also view the action from a third person perspective when holding a turret.
The multiplayer mode is basically everything that was featured in the Multiplayer Beta version released earlier this year (mid May to early June). Although a strong number of match types have been added, such as infection, a human vs. super zombie game, the overall look and feel hasn't really changed.
Of course if something radical does occur, you can save it in the theater mode. The game will hold about your last ten matches, and you can save these videos and go through them to edit them for dramatic effect. It's a nice addition that lets you relive some of your most impressive games, although rewinding and fast-forwarding is rather clunky. You get used to it though.
The final big addition to the Halo series is the forge mode. This is essentially the map eiditor from other games, with one big difference. You are not editing the level geometry, you edit every single item placed in those maps. In Forge mode, the more time you spend in it, the more you can get out of it, and it adds some replay value to the simple split-screen match ups with friends.
Halo 3 provides everything that fans have been waiting about three years for. Bungie has certainly made up for lost time. If you are a fan of Halo, a gamer in the long-run, a person who owns an Xbox 360 and Xbox Live, or anyone who has $60 bucks to spend should go right out and buy Halo 3.