It has a rough start, but quickly pulls together and provides some of the best FPS gameplay available on the 360.
Chunk One: Campaign.
The campaign begins where Master Chief, or Spartan 117, crash lands on Earth ( I think, I apologize if I'm wrong) following the end of Halo 2. The first couple of missions are kind of weak, in my opinion, but after that it is extremely fun. Master Chief fights the Covenant, the Truth, and The Flood in Halo 3. The campaign has nine missions, and a couple of them are very long. It provides from 7-9 hours of gameplay. The campaign's strongest when it is in it's driving sequences, they are fun and don't stretch on for too long. But the BEST part of the campaign is the ending. It is incredibly epic, and also incredibly fun to play over and over again. Also, an important part of the campaign and multiplayer is the addition of power-ups. They range from bubble shields to overshields to turrets to invincibility. They provide more strategy in multiplayer, and more fun within the campaign.
Chunk Two: Multiplayer.
The best part of Halo 3 is definitely the multiplayer. There are about 10 maps to choose from, and all are at least good, and some are awesome. However, maps like Isolation and Valhalla tend to get old after playing them a few times in a day. Some game modes include Slayer(deathmatch), Capture the Flag, and over a dozen other modes. It is extremely fun and I doubt will get old.
Until Halo 4, of course.
Chunk Three: Forge.
Forge is the object editing system where you play god editing, moving, deleting or adding various objects to any of the multiplayer maps. Each object, usually depending on size or rarity, costs different dollar amounts, and each map has it's own budget to place objects. Forge can end up swallowing hours of time experimenting with the physics or just plain screwing around. It's also kind of fun to play Forge with others over Xbox Live, but sometimes people will have conflicts over object placement on the map and leave. You can play multiplayer games on your Forge maps.
Chunk Four: Theater.
Theatre is the Halo lobby in which you can view your previous multiplayer(live or local), campaign, or forge sessions. It records the last thirty( I believe) sessions you participated in. You can fast forward, change angles, and record films. You can also send films to bungie.net. Personally, I think this is the least useful section of Halo 3, but it is still definitely useful.
So, all in all, I think Halo is definitely worth a 50 dollar buy. It is the best game in the Halo series, and now officially one of my favorite games ever.
I have two words to say to Bungie:
THANK YOU.