It's overhyped to hell but it's still a great game!
The graphics are probably the most disappointing aspect of this game, sometimes the game looks cool and other times it's downright ugly. Bungie probably had to sacrifice some eye-candy to achieve a decent framerate on a 4 player XBL Co-Op game... let's not forget some of those outdoor maps are huge.
The sound is great and the game offers a crazy amount of dialogue from your enemies and your AI partners. "Nice shot sir!!" The music is low-key but picks up in certain scripted events.
The gameplay is basically Halo 2 with a new thing called equipments which I barely used. So it's all good.
The main campaign is of adequate length and concludes the trilogy like advertised. It's really fun and you can comeback to it via Co-Op play on XBL or split-screen.
The multiplayer is the meat of this franchise and I can see people still playing this game past the life expectancy of the 360.
In conclusion, I've played better games but Halo 3 is still excellent and I'm glad I had the chance to play it.