When you buy Halo 3 I seriously recommend having a subscription to Xbox Live set up or in the process of being set up. I promise you that you will become bored of story mode very quickly after fanatically completing it on Normal, Heroic, Legendary and Metagame for the gamerscore (if you are a gamerscore whore such as myself). Halo 3 story mode is no different from any other Halo. It is just as mediocre as Halo 2's campaign in my opinion.
The addition of cool new features like Forge and Theatre mode offer players huge oppertunities to play the game they want and that's no just me kissing up to Bungie. You could spend hours altering maps, adding your own touch to the pre-programmed maps as well as the downloadable content or taking cool screenshots and video clips from both story and online gameplay, allowing you to look back at past games and (if you are very enthusiastic about your style) analyse your games and where you keep making mistakes. Halo 3 is also provided with excellent online backup from Bungie.net allowing players to view their character on the site as well as their online stats such as percentages of weapon usage, medal chests displaying the number of medals earned online and Heatmaps showing players where their kills are made and where they die the most in games online.
Online gaming is what really makes Halo 3 one of the best games ever though. Although there are plenty of other games with online capablities Halo has a certain feel about it that just sets it apart from other games in the genre. Maybe it is the huge variety of playlists (new ones added for weekend events), the chance to play Social matches in which players are allowed to have upto 3 guests all playing through one Xbox online or the huge appeal to many to earn the rank of General in Ranked playlists I do not know but one thing I do know is that without Xbox Live I would have gotten rid of Halo 3 months ago.