Been playing Halo 3 on the Xbox recently and can recommend it. Never was a great fan of previous Halo games and only decided to take a look when a relative bought the game for themselves.
After initially letting them burn themselves out on it I snuck it away and played it myself and became pretty wrapped up in the not only the action but also the story. Unlike other games I've played it is pretty easy to play (I ain't too good on button mashing style games) and has a great flow so you don't get bogged down too much.
I think that Microsoft will have a major hit on its hands with Halo 3. Besides it being a great game you gotta see the graphics WOW is all I can say.