Only really good for online play, which still doesn't offer much.
The storyline is really good, it shows the chief and his team finally getting revenge on the covenant who want to destroy the galaxy with the halo rings. Sure, it might sound good now, but after a while, it gets a bit boring to say the least. There's not much more to say other than all the cool, totally awesome things that the chief does in the cutscenes you will never get the experience of which is a real let down.
The online play is probably one of the only reasons this game has such a massive hype. I have tried it and at first it was fantastic! Really, really great! But then I played on and got slightly disappointed by doing the same thing over and over, shoot them, run up to them, bash them, do it again to the next player that comes along. Quite dull really.
The only reason for me giving this game a 7 is something about it which is hard to describe. He's Spartan, what else can I say!? I would recommend this to fans of the Halo series and people who are quite new to shooting games. If you are a Veteran in shooters, turn away from Halo 3, you will be disappointed.