It's a darn great game, but can be quite frustrating.
The multiplayer in Halo 3 is great, it builds on the succes of Halo 2's multiplayer while adding some new experiences. Also the 4 player co-op is one of the funnest experiences ever. The campaign may get kinda confusing sometimes about the story or where to go, but it's all good. It still doesn't live up to Halo: CE's reputation but it comes close. You will have soem fun time trying to get the achievments and the skulls, though you can get them on normal which ultimately ruins the skill of getting them.
My score is 9.5 out of 10 because it combines a great campaign with outstanding multiplayer and other great game types such as forge (which is awesome and you can have so much fun doing crazy sh*t) and it now even has a video replay to watch that sweeet stick you got on that dude who was trash talking. It will keep you going as long as you dont mind being frustrated by the online community and the difficulty of it.
Fantastic game a must have for a hardcore, casual or even a new 360 gamer.