Halo 3 is a must-have for all Xbox 360 owners. The campaign keeps you coming back for more.
The campaign has four difficulties, in case you didn't know, and each one of them is very different. From beating the game alone alot of gamerscore is awarded. Also, skulls on different campaign maps unlock the Hayabusa armor and interesting challenges, or just random bonuses, such as the Grunt B-Day Party.
The four difficulties, Easy, Normal, Heroic, and Legendary are clearly defined as "really easy, easy, challenging, and impossible". When playing on Normal, skulls can be found and unlock cool armor and campain challenges. Beating the game on certain difficulties can unlock certain armor, although most armor is unlocked on Xbox LIVE. If you get all of the achievements in Halo 3, you are rewarded an armor that is the same as the last armor that your Spartan was wearing, but with a sword on your back (just decoration)!
Multiplayer is composed of many main gametypes and can be modified to make new ones. All the customization options in Halo 3 is a huge factor in why the game is so popular. forge also gives you the option to change some of the main multiplayer maps, which is also great.
On Xbox LIVE, new map packs come out all the time and each map pack SHOULD be downloaded in my opinion because if you don't have certain maps, Xbox LIVE can be excluding from some matches. Also, new skulls can be downloaded and almost all of the armor comes from leveling up in Xbox LIVE.
In conclusion, Halo 3 is a must-have for all Xbox 360 owners and if you don't own one now, go out and buy one!