A beautiful, compelling game that immerses you in a great story. All the hype you've heard is TRUE.
Many people say the game is all hype. Well, that hype happens to be exactly true. Bungie did not promise the game to end all games, they just promised an exciting end to the series so many have followed for these last six years. And that's what they gave us. While there is room for other games to defeat it someday, for now, Halo 3 holds the title.
The game has a truly immersive story, bringing an exciting, and truly pleasurable end to the series. Many complain about nostalgia, that it feels like they're playing Halo 1 and 2 again: that's good. They were continuing the previous two, not making a new game from scratch. The AI is great as well. While the marines can't drive to save their lives, they still perform very well in the gunner position and on foot, using well-organized tactics to defeat the Covenant, the antagonists in Halo. Of course, the Covenant are even smarter. The Covenant in this game are much improved from the last. The Brutes are deceptively vicious and, well, brutal, but they are master tacticians, and I have personally seen them perform military maneuvers leading to my defeat at the hands of a Brute and three Grunts.
The multiplayer is revolutionary at least. The same matchmaking system from Halo 2, with improvements, is used in Halo 3. Once again, that's good. If it's not broke, don't fix it. And the Forge system is great, with many options and truly endless possibilities. I play the game actively and come across new maps every time I play.
Of course, no game is without its flaws. The occasional lag is bearable, and will only come up most of the time is if you start making explosions bigger than your hometown. The campaign is great, but admittedly short. Finally, no custom games search and limited Forge can be disappointing, but they really don't subtract that much from the game if you have more than five friends on Live.
Finally, Bungie is very supportive of its community, listening to their complaints and going to the ends of the Ark to fix them, and constantly releasing new maps so the gameplay stays fresh. In just two months, three new maps have been made, one called "the perfect sandbox" by some.
Overall, Halo 3 is an underappreciated game, and was entirely worth the wait. Don't just listen to your CoD4 fan friends (Though there's nothing wrong with that game.) At least rent it and play it for a week before making an opinion, as first impressions aren't always great.