For many people, Halo 3 is the "killer app" of the xbox 360. This is a strong statement as the 360 packs a large library of singleplayer and multiplayer shooters. Indeed, Halo 3 does stand out.
Halo 3 is one of the better looking shooters of 2007-2008. The resolution is sub 1280x720, but antialiasing makes up for it. Beautiful level design and lighting all add to the graphical experience of Halo 3. Also, the game runs at a fairly smooth 30+fps the majority of the time.
Although there is nothing particulary strong about the audio, it works as well as it should. The guns all feature unique and memorable audio, and the voice acting is on par with other good shooters.
Halo 3 features some of the best Halo gameplay bungie has delivered. While Halo and Halo 2 were grossly unbalanced multiplayer games, Halo 3 has been balanced in every aspect to encourage a fair, enjoyable game.
Halo 3's multiplayer is some of the best on the xbox 360. Joining a game is easy, and the party system makes it easy to game with friends. The online performance can be a bit of a gamble, however. Lag can really be a problem with Halo 3 compared to other 360 titles and most certainly PC gaming. Fortunately, Bungie has included various search options for the matchmaking system that help you get matched with well pinged players.
Overall, Halo 3 is a great package for the money. Considering that it comes with a good campaign, level editor and great online functionality, it's hard to deny recommending Halo 3.