This is simply the best shooter in the game. The Halo series ends with a bang.
The game's singleplayer is the most immersive I've seen since Half-Life 2. As the gamer, you're on the edge of your seat the entire ride, just waiting for what's around the corner. The scenes are extremely well laid out, with one level that resembled the poorly-planned Library scene from Combat Evolved. But aside from that level, the entire game is fresh, new, and fairly easy to follow. The new types of vehicles are very beneficial to the gameplay, and they are easy to play. All the controls seem intuitive, and those odds and ends controls are neatly displayed in the HUD. As a long-time PC fanatic, I have had very limited experience with the XBOX 360 controller, and Halo 3 was the first game that I had played through on my own on the system.
But what really shines is the multiplayer. When the system is up and running, you can get immersed in the gameplay for hours without realizing it. The way Bungie has it set up is nice to noobs like myself, so that you can play with and against others who are new to the game. The ranking system is quite nice, and it's easy to track down friends.
Between both those modes, the audio, again, is superbly added, and the graphics are believable, with very little load time. I experienced no framerate issues in the first 20hours of playing.
If you haven't bought this game, or have a friend that has loaned it to you, you are truly missing one of the greatest games of this generation.